SIL: Force Clang-imported protocol conformances to get deserialized when used.

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
- During deserialization, when the SILLinker walks a loaded module, ensure that all shared conformances get deserialized, including those from ApplyInsts and inherited/associated type protocol requirements.

Fixes rdar://problem/38687726.

include/swift/SIL/SILModule.h

@@ -708,6 +708,11 @@ public:
   bool isDefaultAtomic() const {
     return ! getOptions().AssumeSingleThreaded;
   }
+  
+  /// Returns true if SIL entities associated with declarations in the given
+  /// declaration context ought to be serialized as part of this module.
+  bool shouldSerializeEntitiesAssociatedWithDeclContext(const DeclContext *DC)
+    const;
 };
 
 inline llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, const SILModule &M){