When module umbrellas change, rebuild them

With the VFS, it is easy to hit modified umbrellas by overriding the
umbrella header, and what we want is to rebuild, not to fail.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205975 91177308-0d34-0410-b5e6-96231b3b80d8