Serialize local types and provide a lookup API

rdar://problem/18295292

Locally scoped type declarations were previously not serialized into the
module, which meant that the debugger couldn't reason about the
structure of instances of those types.

Introduce a new mangling for local types:
[file basename MD5][counter][identifier]
This allows the demangle node's data to be used directly for lookup
without having to backtrack in the debugger.

Local decls are now serialized into a LOCAL_TYPE_DECLS table in the
module, which acts as the backing hash table for looking up
[file basename MD5][counter][identifier] -> DeclID mappings.

New tests:
* swift-ide-test mode for testing the demangle/lookup/mangle lifecycle
of a module that contains local decls
* mangling
* module merging with local decls

Swift SVN r24426

include/swift/Serialization/SerializedModuleLoader.h

@@ -166,6 +166,10 @@ public:
                            DeclName name, NLKind lookupKind,
                            SmallVectorImpl<ValueDecl*> &results) const override;
 
+  virtual TypeDecl *lookupLocalType(llvm::StringRef FileHash,
+                                    unsigned int LocalDiscriminator,
+                                    llvm::StringRef Name) const override;
+
   virtual OperatorDecl *lookupOperator(Identifier name,
                                        DeclKind fixity) const override;
 
@@ -184,6 +188,8 @@ public:
 
   virtual void getTopLevelDecls(SmallVectorImpl<Decl*> &results) const override;
 
+  virtual void getLocalTypeDecls(SmallVectorImpl<Decl *> &results) const override;
+
   virtual void getDisplayDecls(SmallVectorImpl<Decl*> &results) const override;
 
   virtual void
@@ -194,6 +200,7 @@ public:
   collectLinkLibraries(Module::LinkLibraryCallback callback) const override;
 
   Identifier getDiscriminatorForPrivateValue(const ValueDecl *D) const override;
+  Identifier getDiscriminatorForLocalValue(const ValueDecl *D) const override;
 
   virtual StringRef getFilename() const override;