[CrashReproducer] Change module map callback signature. NFC

Use a StringRef instead of a FileEntry in the moduleMapAddHeader
callback to allow more flexibility on what to collect on further
patches. This changes the interface I introduced in r264971.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@268819 91177308-0d34-0410-b5e6-96231b3b80d8