Avoid emitting redundant or unusable directories in DIFile metadata entries.

As discussed on llvm-dev recently, Clang currently emits redundant
directories in DIFile entries, such as

  .file	     1 "/Volumes/Data/llvm" "/Volumes/Data/llvm/tools/clang/test/CodeGen/debug-info-abspath.c"

This patch looks at any common prefix between the compilation
directory and the (absolute) file path and strips the redundant
part. More importantly it leaves the compilation directory empty if
the two paths have no common prefix.

After this patch the above entry is (assuming a compilation dir of "/Volumes/Data/llvm/_build"):

  .file 1 "/Volumes/Data/llvm" "tools/clang/test/CodeGen/debug-info-abspath.c"

When building the FileCheck binary with debug info, this patch makes
the build artifacts ~1kb smaller.

Differential Revision: https://reviews.llvm.org/D55085

llvm-svn: 348154