Eliminate CIndexer::getClangPath(), since libclang no longer depends

on the presence of a 'clang' executable. Simplify
CIndexer::getClangResourcesPath() a bit.

Patch up the CMake makefiles to install headers into two locations in
the build tree, for those silly cases where 'clang' will end up
looking into the wrong build directory for headers.

llvm-svn: 116260