[PCH] Fix a regression that r139441 introduced (decls were getting passed

to the consumer without being fully deserialized).

The regression was on compiling boost.python and it was too difficult to get a reduced
test case unfortunately.

Also modify the logic of how objc methods are getting passed to the consumer;
codegen depended on receiving objc methods before the implementation decl.
Since the interesting objc methods are ones with a body and such methods only
exist inside an ObjCImplDecl, deserialize and pass to consumer all the methods
of ObCImplDecl when we see one.

Fixes http://llvm.org/PR10922 & rdar://10117105.

llvm-svn: 139644