[Serialization] Collapse indirection in DeclContextID

...by making it a tagged union of either a DeclID or a
LocalDeclContextID. This should lead to smaller module files and be
slightly more efficient to deserialize, and also means that every
AST entity kind is serialized in exactly one way, which allows for
the following commit's refactoring.

lib/Serialization/ModuleFile.cpp

@@ -874,10 +874,6 @@ bool ModuleFile::readIndexBlock(llvm::BitstreamCursor &cursor) {
         assert(blobData.empty());
         allocateBuffer(Decls, scratch);
         break;
-      case index_block::DECL_CONTEXT_OFFSETS:
-        assert(blobData.empty());
-        allocateBuffer(DeclContexts, scratch);
-        break;
       case index_block::TYPE_OFFSETS:
         assert(blobData.empty());
         allocateBuffer(Types, scratch);