Revert the r348352 "[clang] - Simplify tools::SplitDebugName."

This partially reverts the r348352 (https://reviews.llvm.org/D55006)
because of https://bugs.llvm.org/show_bug.cgi?id=41161.

I did not revert the test case file because it passes fine now.

llvm-svn: 357061