First step to allowing the resource directory of Clang to be adjusted for

First step to allowing the resource directory of Clang to be adjusted for
strange packaging environments. The primary result of this is to expose
a (normally empty) CLANG_RESOURCE_DIR string in the autoconf and CMake builds.
This will in turn be used by a subsequent commit to Clang.

Regenerated configure and config.h.in thanks to Nick. =D

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@116802 91177308-0d34-0410-b5e6-96231b3b80d8