IRGen: Deserialize SIL witness tables and shared-linkage definitions by need.

Code may end up indirectly using a witness table for a Clang-imported type by inlining code that used the conformance from another module, in which case we need to ensure we have a local definition at hand in the inlining module so we can have something to link against independently. This needs to be fixed from both sides:

- During serialization, serialize not only witness tables from the current module, but from Clang-imported modules too, so that their definitions can be used by other modules that inline code from the current module
- During IRGen, when we emit a reference to a SILWitnessTable or SILFunction declaration with shared linkage, attempt to deserialize the definition on demand

Fixes rdar://problem/38687726.

lib/IRGen/IRGenModule.cpp

@@ -24,6 +24,7 @@
 #include "swift/IRGen/Linking.h"
 #include "swift/Runtime/RuntimeFnWrappersGen.h"
 #include "swift/Runtime/Config.h"
+#include "swift/SIL/FormalLinkage.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/Basic/CharInfo.h"
 #include "clang/Basic/TargetInfo.h"
@@ -658,6 +659,9 @@ llvm::Module *IRGenModule::releaseModule() {
 }
 
 bool IRGenerator::canEmitWitnessTableLazily(SILWitnessTable *wt) {
+  if (LazilyEmittedWitnessTables.count(wt))
+    return true;
+
   if (Opts.UseJIT)
     return false;
 
@@ -685,6 +689,21 @@ void IRGenerator::addLazyWitnessTable(const ProtocolConformance *Conf) {
         LazilyEmittedWitnessTables.insert(wt).second) {
       LazyWitnessTables.push_back(wt);
     }
+  // Shared-linkage protocol conformances may not have been deserialized yet
+  // if they were used by inlined code. See if we can deserialize it now.
+  } else {
+    auto linkage =
+      getLinkageForProtocolConformance(Conf->getRootNormalConformance(),
+                                       ForDefinition);
+    if (hasSharedVisibility(linkage)) {
+      SIL.createWitnessTableDeclaration(const_cast<ProtocolConformance*>(Conf),
+                                        linkage);
+      SILWitnessTable *wt = SIL.lookUpWitnessTable(Conf);
+      if (wt && wt->isDefinition()
+          && LazilyEmittedWitnessTables.insert(wt).second) {
+        LazyWitnessTables.push_back(wt);
+      }
+    }
   }
 }