Use a SmallString buffer instead of a std::string for debug info path lookup. NFC.

Use a SmallString buffer instead of a std::string for debug info path lookup.  NFC.

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237823 91177308-0d34-0410-b5e6-96231b3b80d8