Provide bootstrap support to build only llvm+clang when using cmake.

Where a source tree is complete with lld, lldb and polly, it may not be possible to use cmake to configure build scripts if the host compiler it not capable of compiling these sub-projects.  This change makes it possible to first build a bootstrap clang compiler when can then be used to build a complete llvm toolchain.  An example bootstrap build sequence could be as follows:

$ mkdir bootstrap
$ cd bootstrap
$ cmake -G 'Unix Makefiles'
        -DCMAKE_BUILD_TYPE:STRING=Release
        -DCMAKE_PREFIX_PATH:STRING=$(pwd)
        -DLLVM_TARGETS_TO_BUILD:STRING=host
        -DLLVM_INCLUDE_TOOLS:STRING=bootstrap-only
        ../source
$ make clang  # build clang only for host

$ cd ..
$ export CC=$(realpath bootstrap/bin)/clang
$ export CXX=$(realpath bootstrap/bin)/clang++

$ mkdir final
$ cd final
$ cmake -G 'Unix Makefiles' ../source
$ make all check-all