[serialization] Add support for FunctionType.

The only tricky thing here is that the calling convention enum needs to be
stable in the module format, so it's marshalled in and out by helper
functions when crossing serialization boundaries.

Swift SVN r5699

lib/Serialization/ModuleFile.cpp

@@ -302,6 +302,26 @@ Decl *ModuleFile::getDecl(DeclID DID) {
   return declOrOffset.get<Decl *>();
 }
 
+/// Translate from the Serialization calling convention enum values to the AST
+/// strongly-typed enum.
+///
+/// The former is guaranteed to be stable, but may not reflect this version of
+/// the AST.
+static Optional<swift::AbstractCC> getActualCC(uint8_t cc) {
+  switch (cc) {
+#define CASE(THE_CC) \
+  case static_cast<uint8_t>(serialization::AbstractCC::THE_CC): \
+    return swift::AbstractCC::THE_CC;
+  CASE(C)
+  CASE(ObjCMethod)
+  CASE(Freestanding)
+  CASE(Method)
+#undef CASE
+  default:
+    return Nothing;
+  }
+}
+
 Type ModuleFile::getType(TypeID TID) {
   if (TID == 0)
     return Type();
@@ -411,6 +431,31 @@ Type ModuleFile::getType(TypeID TID) {
     break;
   }
 
+  case decls_block::FUNCTION_TYPE: {
+    TypeID inputID;
+    TypeID resultID;
+    uint8_t rawCallingConvention;
+    bool autoClosure;
+    bool thin;
+    bool blockCompatible;
+
+    decls_block::FunctionTypeLayout::readRecord(scratch, inputID, resultID,
+                                                rawCallingConvention,
+                                                autoClosure, thin,
+                                                blockCompatible);
+    auto callingConvention = getActualCC(rawCallingConvention);
+    if (!callingConvention.hasValue()) {
+      error();
+      return nullptr;
+    }
+
+    typeOrOffset = FunctionType::get(getType(inputID), getType(resultID),
+                                     autoClosure, blockCompatible, thin,
+                                     callingConvention.getValue(),
+                                     ModuleContext->Ctx);
+    break;
+  }
+
   default:
     // We don't know how to deserialize this kind of type.
     error();