[clang-tidy] Pass absolute path to OptionsProvider::getOptions/getRawOptions.

Summary:
Although there is no guarantee of getOptions/getRawOptions receiving an
absolute path, we try to make it if possible. So FileOptionProvider subclasses
don't have to convert the path to an absolute path.

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D22154

llvm-svn: 275051