[clang-format] Add option to specify explicit config file

Summary:
This diff extends the -style=file option to allow a config file to be specified explicitly. This is useful (for instance) when adding IDE commands to reformat code to a personal style.

Reviewers: djasper, ioeric, krasimir, MyDeveloperDay

Reviewed by: MyDeveloperDay

Contributed by: tnorth

Subscribers: cfe-commits, lebedev.ri, MyDeveloperDay, klimek, sammccall, mitchell-stellar

Tags: #clang, #clang-format

Differential Revision: https://reviews.llvm.org/D72326