Summary:
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.
Reviewers: benlangmuir, v.g.vassilev
Reviewed By: v.g.vassilev
Subscribers: mgorny, cfe-commits, v.g.vassilev
Differential Revision: https://reviews.llvm.org/D37416
llvm-svn: 313049