[clang-format] New API guessLanguage()

Summary:
For clients which don't have a filesystem, calling getStyle() doesn't
make much sense (there's no .clang-format files to search for).

In this diff, I hoist out the language-guessing logic from getStyle()
and move it into a new API guessLanguage().

I also added support for guessing the language of files which have no
extension (they could be C++ or ObjC).

Test Plan: New tests added. Ran tests with:
  % make -j12 FormatTests && ./tools/clang/unittests/Format/FormatTests

Reviewers: jolesiak, krasimir

Reviewed By: jolesiak, krasimir

Subscribers: klimek, cfe-commits, sammccall

Differential Revision: https://reviews.llvm.org/D43522

llvm-svn: 325691