[tests] Fix the FileManagerTest getVirtualFile test on Windows
[tests] Fix the FileManagerTest getVirtualFile test on Windows

Summary: The test passes on Windows only when it is executed on the C: drive. If the build and tests run on a different drive, the test is currently failing.

Reviewers: kadircet, asmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D55451

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@348665 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang