Make FormatStyle.GetStyleOfFile test work on MSVC

Modify getStyle to use vfs::FileSystem::makeAbsolute just like FS.addFile does,
rather than sys::fs::make_absolute. The latter gets the CWD from the platform,
while the former expects it to be set by the client, causing a mismatch when
converting relative paths to absolute.

Differential Revision: https://reviews.llvm.org/D27971

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@290319 91177308-0d34-0410-b5e6-96231b3b80d8