[clang] - Simplify tools::SplitDebugName.

This should be NFC change.

SplitDebugName recently started to accept the `Output` that
can be used to simplify the logic a bit, also it
seems that code in SplitDebugName that uses
OPT_fdebug_compilation_dir is simply dead.

Differential revision: https://reviews.llvm.org/D54576

llvm-svn: 347035