[Serialization] Improve extensions of nested types with the same name (#7397)

Previously looking up an extension would result in all extensions for
types with the same name (nested or not) being deserialized; this
could even bring in base types that had not been deserialized yet. Add
in a string to distinguish an extension's base type; in the top-level
case this is just a module name, but for nested types it's a full
mangled name.

This is a little heavier than I'd like it to be, since it means we
mangle names and then throw them away, and since it means there's a
whole bunch of extra string data in the module just for uniquely
identifying a declaration. But it's correct, and does less work than
before, and fixes a circularity issue with a nested type A.B.A that
apparently used to work.

https://bugs.swift.org/browse/SR-3915

include/swift/Serialization/ModuleFile.h

@@ -312,6 +312,10 @@ private:
   using SerializedDeclTable =
       llvm::OnDiskIterableChainedHashTable<DeclTableInfo>;
 
+  class ExtensionTableInfo;
+  using SerializedExtensionTable =
+      llvm::OnDiskIterableChainedHashTable<ExtensionTableInfo>;
+
   class LocalDeclTableInfo;
   using SerializedLocalDeclTable =
       llvm::OnDiskIterableChainedHashTable<LocalDeclTableInfo>;
@@ -323,9 +327,9 @@ private:
   std::unique_ptr<SerializedDeclTable> TopLevelDecls;
   std::unique_ptr<SerializedDeclTable> OperatorDecls;
   std::unique_ptr<SerializedDeclTable> PrecedenceGroupDecls;
-  std::unique_ptr<SerializedDeclTable> ExtensionDecls;
   std::unique_ptr<SerializedDeclTable> ClassMembersByName;
   std::unique_ptr<SerializedDeclTable> OperatorMethodDecls;
+  std::unique_ptr<SerializedExtensionTable> ExtensionDecls;
   std::unique_ptr<SerializedLocalDeclTable> LocalTypeDecls;
   std::unique_ptr<SerializedNestedTypeDeclsTable> NestedTypeDecls;
 
@@ -446,6 +450,11 @@ private:
   std::unique_ptr<ModuleFile::SerializedObjCMethodTable>
   readObjCMethodTable(ArrayRef<uint64_t> fields, StringRef blobData);
 
+  /// Read an on-disk local decl hash table stored in
+  /// index_block::ExtensionTableLayout format.
+  std::unique_ptr<SerializedExtensionTable>
+  readExtensionTable(ArrayRef<uint64_t> fields, StringRef blobData);
+
   /// Read an on-disk local decl hash table stored in
   /// index_block::NestedTypeDeclsLayout format.
   std::unique_ptr<SerializedNestedTypeDeclsTable>