Change Path::filename_pos() to skip the drive letter.

Change Path::filename_pos() to skip the drive letter.

For Windows, filename_pos() tries to find the filename by
searching for separators after the last :.  Instead, it should
really check for the only location that a : is valid, which is
in the second character, and search for separators after that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228874 91177308-0d34-0410-b5e6-96231b3b80d8