Parse .h files as objective-c++ if we don't have a compile command.

Summary: This makes C++/objC not totally broken, without hurting C files too much.

Reviewers: ilya-biryukov

Subscribers: klimek, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D45442

llvm-svn: 330418