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

lib/Parse/ParseDecl.cpp

@@ -1371,8 +1371,12 @@ void Parser::setLocalDiscriminator(ValueDecl *D) {
   if (!CurLocalContext || !D->getDeclContext()->isLocalContext())
     return;
 
+  if (auto type = dyn_cast<TypeDecl>(D))
+    SF.LocalTypeDecls.push_back(type);
+
   Identifier name = D->getName();
-  unsigned discriminator = CurLocalContext->claimNextNamedDiscriminator(name);
+  auto SF = D->getDeclContext()->getParentSourceFile();
+  auto discriminator = SF->claimNextNamedDiscriminator(name);
   D->setLocalDiscriminator(discriminator);
 }