[Basic] Update CMakeLists.txt to handle repo

Summary:
The find_first_existing_file and find_first_existing_vc_file macros in
lib/Basic/CMakeLists.txt are removed. The macros are also defined in
{LLVM}/cmake/modules/AddLLVM.cmake for the same purpose. This change
serves the following 2 objectives:

    1. To remove the redundant code in clang to use the same
       macros in llvm,
    2. The macros in AddLLVM.cmake can also handle repo for
       displaying correct version information.

Reviewers: jordan_rose, cfe-commits, modocache, hintonda

Reviewed By: hintonda

Subscribers: mgorny

Differential Revision: https://reviews.llvm.org/D35533

llvm-svn: 312865