[clang] - Simplify tools::SplitDebugName.

This is an updated version of the D54576, which was reverted.

Problem was that SplitDebugName calls the InputInfo::getFilename
which asserts if InputInfo given is not of type Filename:

const char *getFilename() const {
  assert(isFilename() && "Invalid accessor.");
  return Data.Filename;
}
At the same time at that point, it can be of type Nothing and
we need to use getBaseInput(), like original code did.

Differential revision: https://reviews.llvm.org/D55006

llvm-svn: 348352