This code appends the filename to the directory then looks that up in a StringMap. We should be using the existing Twine::toStringRef method instead of Twine::str() as most times we'll succeed in the lookup.
Its possible that we should also consider allowing StringMap to lookup a key using a Twine in addition to a StringRef but that would complicate the code with little known benefit above and beyond this change.
This saves 170k temporary allocations when running llc on the verify_use_list_order bitcode with debug info for x86.
llvm-svn: 237823