Resolve build bot problems in unittests/Format/FormatTest.cpp

Summary:
Make the new GetStyleWithEmptyFileName test case independent
of the file system used when running the test. Since the
test is supposed to use the fallback "Google" style we now
use a InMemoryFileSystem to make sure that we do not accidentaly
find a .clang-format file in the real file system. That could
for example happen when having the build directory inside the
llvm och clang repo (as there is a .clang-format file inside
the repos).

Reviewers: vsapsai, jolesiak, krasimir, benhamilton

Reviewed By: krasimir

Subscribers: uabelho, twoh, klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D43732

llvm-svn: 326086