Change getString() to return Optional<StringRef>, and change
lookupFilename() to return an empty string if either one of the prefix
and suffix can't be found.

This is a more robust follow-up to r261461, but it's still not entirely
satisfactory.  Ideally we'd report that the header map is corrupt;
perhaps something for a follow-up.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@261596 91177308-0d34-0410-b5e6-96231b3b80d8