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/SIL/SILModule.cpp

@@ -22,6 +22,7 @@
 #include "Linker.h"
 #include "swift/SIL/SILVisitor.h"
 #include "swift/SIL/SILValue.h"
+#include "swift/ClangImporter/ClangModule.h"
 #include "llvm/ADT/FoldingSet.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringSwitch.h"
@@ -781,6 +782,23 @@ bool SILModule::isNoReturnBuiltinOrIntrinsic(Identifier Name) {
   }
 }
 
+bool SILModule::
+shouldSerializeEntitiesAssociatedWithDeclContext(const DeclContext *DC) const {
+  // Serialize entities associated with this module's associated context.
+  if (DC->isChildContextOf(getAssociatedContext())) {
+    return true;
+  }
+  
+  // Serialize entities associated with clang modules, since other entities
+  // may depend on them, and someone who deserializes those entities may not
+  // have their own copy.
+  if (isa<ClangModuleUnit>(DC->getModuleScopeContext())) {
+    return true;
+  }
+  
+  return false;
+}
+
 /// Returns true if it is the OnoneSupport module.
 bool SILModule::isOnoneSupportModule() const {
   return getSwiftModule()->getName().str() == SWIFT_ONONE_SUPPORT;