[codeview] Merge file checksum entries for DIFiles with the same absolute path

[codeview] Merge file checksum entries for DIFiles with the same absolute path

Change the map key from DIFile* to the absolute path string. Computing
the absolute path isn't expensive because we already have a map that
caches the full path keyed on DIFile*.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317041 91177308-0d34-0410-b5e6-96231b3b80d8