[Serialization] Preparations for removal of getName on ValueDecl

With the introduction of special decl names, `Identifier getName()` on
`ValueDecl` will be removed and pushed down to nominal declarations
whose name is guaranteed not to be special. Prepare for this by calling
to `DeclBaseName getBaseName()` instead where appropriate.

lib/Serialization/ModuleFile.cpp

@@ -314,13 +314,14 @@ namespace {
 class ModuleFile::DeclTableInfo {
 public:
   using internal_key_type = StringRef;
-  using external_key_type = Identifier;
+  using external_key_type = DeclBaseName;
   using data_type = SmallVector<std::pair<uint8_t, DeclID>, 8>;
   using hash_value_type = uint32_t;
   using offset_type = unsigned;
 
   internal_key_type GetInternalKey(external_key_type ID) {
-    return ID.str();
+    // TODO: Handle special names
+    return ID.getIdentifier().str();
   }
 
   hash_value_type ComputeHash(internal_key_type key) {
@@ -338,6 +339,7 @@ public:
   }
 
   static internal_key_type ReadKey(const uint8_t *data, unsigned length) {
+    // TODO: Handle special names
     return StringRef(reinterpret_cast<const char *>(data), length);
   }