Added support for different VFSs in format::getStyle.

Summary:
Previously, format::getStyle assumes that the given file resides in
the real file system, which prevents the use of virtual file system in testing etc.
This patch adds a parameter in format::getStyle interface so that users can specify
the right file system. By default, the file system is the real file system.

Reviewers: djasper, klimek

Subscribers: cfe-commits, klimek

Differential Revision: http://reviews.llvm.org/D18399

llvm-svn: 264253