[CMake] Support overriding binary install directory

This patch adds a new option LLVM_TOOLS_INSTALL_DIR which allows customizing the location executables and symlinks get installed to. This adds the functionality provided by autoconf's --bindir flag.

This patch is based on patches from and collaboration with Tony Kelman, and replaces http://reviews.llvm.org/D20934.

llvm-svn: 272200