of the file name. This is consistent with how other HeaderSearchOptions
are handled.

Due to the other inputs of the module hash (revision number) this is not
really testable in a meaningful way.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@257520 91177308-0d34-0410-b5e6-96231b3b80d8