[clang-format] New API guessLanguage()
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