Adapt gcov to changes in CFE.
Adapt gcov to changes in CFE.

The clang frontend no longer emits the current working directory for
DIFiles containing an absolute path in the filename: and will move the
common prefix between current working directory and the file into the
directory: component.

This fixes the GCOV tests in compiler-rt that were broken by the Clang
change.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@348203 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm