Validate user headers even if -fmodules-validate-once-per-build-session

is enabled. Unlike system headers, we want to be more careful about
modifications to user headers, because it's still easy to edit a header
while you're building.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221634 91177308-0d34-0410-b5e6-96231b3b80d8