[LIT] Fix testing out-of-tree Clang builds

Summary:
Currently, LIT configures the LLVM binary path before the Clang binary path. However this breaks testing out-of-tree Clang builds (where the LLVM binary path includes a copy of Clang).

This patch reverses the order of the paths when looking for Clang, putting the Clang binary directory first.

Reviewers: zturner, beanz, chapuni, modocache, EricWF

Reviewed By: EricWF

Subscribers: mgorny, cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D40217

llvm-svn: 318607