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/ModuleFormat.h

@@ -51,7 +51,7 @@ const uint16_t VERSION_MAJOR = 0;
 /// To ensure that two separate changes don't silently get merged into one
 /// in source control, you should also update the comment to briefly
 /// describe what change you made.
-const uint16_t VERSION_MINOR = 166; // Last change: non-native owning addressor
+const uint16_t VERSION_MINOR = 167; // Last change: local type serialization
 
 using DeclID = Fixnum<31>;
 using DeclIDField = BCFixed<31>;
@@ -1242,6 +1242,8 @@ namespace index_block {
     /// Objective-C selectors to the methods/initializers/properties/etc. that
     /// produce Objective-C methods.
     OBJC_METHODS,
+
+    LOCAL_TYPE_DECLS,
   };
 
   using OffsetsLayout = BCGenericRecordLayout<