Implement support for code completion of an Objective-C message send to

Implement support for code completion of an Objective-C message send to
"id" or an expression of type "id". In these cases, we produce a list
of all of the (class or instance) methods, respectively, that we know about.

Note that this implementation does not yet work well with precompiled
headers; that's coming soon.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@100528 91177308-0d34-0410-b5e6-96231b3b80d8