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

This reverts commit 228874.  For some reason users reported
seeing Clang taking up 25+GB of memory and bringing down
machines with this change.  Reverting until we figure it out.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@228890 91177308-0d34-0410-b5e6-96231b3b80d8

dslab-epfl/asap,llvm-mirror/llvm,apple/swift-llvm,dslab-epfl/asap,llvm-mirror/llvm,GPUOpen-Drivers/llvm,dslab-epfl/asap,dslab-epfl/asap,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,dslab-epfl/asap,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,dslab-epfl/asap,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,dslab-epfl/asap,apple/swift-llvm,llvm-mirror/llvm