[Format] Improve ObjC header guessing heuristic

Summary:
This improves upon the previous Objective-C header guessing heuristic
from rC320479.

Now, we run the lexer on C++ header files and look for Objective-C
keywords and syntax. We also look for Foundation types.

Test Plan: make -j12 FormatTests && ./tools/clang/unittests/Format/FormatTests

Reviewers: jolesiak, krasimir

Reviewed By: jolesiak

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D42135

llvm-svn: 322690