The CompilerInstance should create its default VFS from its CompilerInvocation. Right now the
user has to manually create the VFS before creating the FileManager even though
-ivfsoverlay file.yaml was passed via the CompilerInvocation (which is exactly how we worked
around this issue in FrontendAction.cpp so far).
This patch uses the invocation's VFS by default and also tests this behavior now from the
point of view of a program that uses the clang API.