[cmake] Install 'clang-cpp' symlink

Install the 'clang-cpp' symlink used to spawn the preprocessor. The code
handling this suffix is already included in Driver. FreeBSD is already
creating such a symlink in ports, and a similar one was requested
by Gentoo/FreeBSD team. The goal is to handle software that takes a C
preprocessor via a variable but does not handle passing options
correctly (i.e. 'clang -E' does not work).

Bug: https://bugs.gentoo.org/478810

Differential Revision: https://reviews.llvm.org/D25161

llvm-svn: 283075