Update for LLVM API change

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@222304 91177308-0d34-0410-b5e6-96231b3b80d8
Remove uses of StringMap::GetOrCreateValue in favor of stl-compatible API usage

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@222305 91177308-0d34-0410-b5e6-96231b3b80d8