Reapply "Avoid emitting redundant or unusable directories in DIFile metadata entries.""

This reverts commit r348280 and reapplies D55085 without modifications.

Original commit message:

    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: 348513