Implement customizable Sendable conformance diagnostics.

Rework Sendable checking to be completely based on "missing"
conformances, so that we can individually diagnose missing Sendable
conformances based on both the module in which the conformance check
happened as well as where the type was declared. The basic rules here
are to only diagnose if either the module where the non-Sendable type
was declared or the module where it was checked was compiled with a
mode that consistently diagnoses `Sendable`, either by virtue of
being Swift 6 or because `-warn-concurrency` was provided on the
command line. And have that diagnostic be an error in Swift 6 or
warning in Swift 5.x.

There is much tuning to be done here.

lib/AST/Module.cpp

@@ -489,6 +489,7 @@ ModuleDecl::ModuleDecl(Identifier name, ASTContext &ctx,
   Bits.ModuleDecl.IsNonSwiftModule = 0;
   Bits.ModuleDecl.IsMainModule = 0;
   Bits.ModuleDecl.HasIncrementalInfo = 0;
+  Bits.ModuleDecl.IsConcurrencyChecked = 0;
 }
 
 ImplicitImportList ModuleDecl::getImplicitImports() const {
@@ -978,6 +979,21 @@ static bool shouldCreateMissingConformances(ProtocolDecl *proto) {
   return proto->isSpecificProtocol(KnownProtocolKind::Sendable);
 }
 
+ProtocolConformanceRef ProtocolConformanceRef::forMissingOrInvalid(
+    Type type, ProtocolDecl *proto) {
+  // Introduce "missing" conformances when appropriate, so that type checking
+  // (and even code generation) can continue.
+  ASTContext &ctx = proto->getASTContext();
+  if (shouldCreateMissingConformances(proto)) {
+    return ProtocolConformanceRef(
+        ctx.getBuiltinConformance(
+          type, proto, GenericSignature(), { },
+          BuiltinConformanceKind::Missing));
+  }
+
+  return ProtocolConformanceRef::forInvalid();
+}
+
 ProtocolConformanceRef ModuleDecl::lookupConformance(Type type,
                                                      ProtocolDecl *protocol,
                                                      bool allowMissing) {
@@ -1007,23 +1023,6 @@ ProtocolConformanceRef ModuleDecl::lookupConformance(Type type,
   return result;
 }
 
-/// Retrieve an invalid or missing conformance, as appropriate, when a
-/// legitimate conformance doesn't exist.
-static ProtocolConformanceRef getInvalidOrMissingConformance(
-    Type type, ProtocolDecl *proto) {
-  // Introduce "missing" conformances when appropriate, so that type checking
-  // (and even code generation) can continue.
-  ASTContext &ctx = proto->getASTContext();
-  if (shouldCreateMissingConformances(proto)) {
-    return ProtocolConformanceRef(
-        ctx.getBuiltinConformance(
-          type, proto, GenericSignature(), { },
-          BuiltinConformanceKind::Missing));
-  }
-
-  return ProtocolConformanceRef::forInvalid();
-}
-
 /// Synthesize a builtin tuple type conformance to the given protocol, if
 /// appropriate.
 static ProtocolConformanceRef getBuiltinTupleTypeConformance(
@@ -1083,7 +1082,7 @@ static ProtocolConformanceRef getBuiltinTupleTypeConformance(
         ctx.getSpecializedConformance(type, genericConformance, subMap));
   }
 
-  return getInvalidOrMissingConformance(type, protocol);
+  return ProtocolConformanceRef::forMissingOrInvalid(type, protocol);
 }
 
 /// Whether the given function type conforms to Sendable.
@@ -1116,7 +1115,7 @@ static ProtocolConformanceRef getBuiltinFunctionTypeConformance(
                                   BuiltinConformanceKind::Synthesized));
   }
 
-  return getInvalidOrMissingConformance(type, protocol);
+  return ProtocolConformanceRef::forMissingOrInvalid(type, protocol);
 }
 
 /// Synthesize a builtin metatype type conformance to the given protocol, if
@@ -1131,7 +1130,7 @@ static ProtocolConformanceRef getBuiltinMetaTypeTypeConformance(
                                   BuiltinConformanceKind::Synthesized));
   }
 
-  return getInvalidOrMissingConformance(type, protocol);
+  return ProtocolConformanceRef::forMissingOrInvalid(type, protocol);
 }
 
 /// Synthesize a builtin type conformance to the given protocol, if
@@ -1146,7 +1145,7 @@ static ProtocolConformanceRef getBuiltinBuiltinTypeConformance(
                                   BuiltinConformanceKind::Synthesized));
   }
 
-  return getInvalidOrMissingConformance(type, protocol);
+  return ProtocolConformanceRef::forMissingOrInvalid(type, protocol);
 }
 
 ProtocolConformanceRef
@@ -1184,7 +1183,7 @@ LookupConformanceInModuleRequest::evaluate(
         return ProtocolConformanceRef(protocol);
     }
 
-    return getInvalidOrMissingConformance(type, protocol);
+    return ProtocolConformanceRef::forMissingOrInvalid(type, protocol);
   }
 
   // An existential conforms to a protocol if the protocol is listed in the
@@ -1193,7 +1192,7 @@ LookupConformanceInModuleRequest::evaluate(
   if (type->isExistentialType()) {
     auto result = mod->lookupExistentialConformance(type, protocol);
     if (result.isInvalid())
-      return getInvalidOrMissingConformance(type, protocol);
+      return ProtocolConformanceRef::forMissingOrInvalid(type, protocol);
     return result;
   }
 
@@ -1231,13 +1230,13 @@ LookupConformanceInModuleRequest::evaluate(
 
   // If we don't have a nominal type, there are no conformances.
   if (!nominal || isa<ProtocolDecl>(nominal))
-    return getInvalidOrMissingConformance(type, protocol);
+    return ProtocolConformanceRef::forMissingOrInvalid(type, protocol);
 
   // Find the (unspecialized) conformance.
   SmallVector<ProtocolConformance *, 2> conformances;
   if (!nominal->lookupConformance(protocol, conformances)) {
     if (!protocol->isSpecificProtocol(KnownProtocolKind::Sendable))
-      return getInvalidOrMissingConformance(type, protocol);
+      return ProtocolConformanceRef::forMissingOrInvalid(type, protocol);
 
     // Try to infer Sendable conformance.
     GetImplicitSendableRequest cvRequest{nominal};
@@ -1246,7 +1245,7 @@ LookupConformanceInModuleRequest::evaluate(
       conformances.clear();
       conformances.push_back(conformance);
     } else {
-      return getInvalidOrMissingConformance(type, protocol);
+      return ProtocolConformanceRef::forMissingOrInvalid(type, protocol);
     }
   }