[clang-format] Adds a FormatStyleSet

Summary:
This patch adds a FormatStyleSet for storing per-language FormatStyles for the
purposes of formatting code blocks inside the main code.

Reviewers: bkramer

Reviewed By: bkramer

Subscribers: klimek, djasper, bkramer, cfe-commits

Differential Revision: https://reviews.llvm.org/D41487

llvm-svn: 322479