[DWARF] Make sure file entry for artificial functions has an MD5 checksum

The DIFile cache was keyed on a string pointer instead of string content,
which was causing misses and resulted in an entry without a checksum.
In DWARF v5 if any checksum is missing, we can't write any to the output
file, so this had consequences.

Fixes https://github.com/llvm/llvm-project/issues/63955

Differential revision: https://reviews.llvm.org/D155991