Add CLANG_BUILD_TOOLS as a clang counterpart for LLVM_BUILD_TOOLS

LLVM_BUILD_TOOLS is a boolean variable that controls whether or not generated
targets for llvm tools are built by the "all" target. CLANG_BUILD_TOOLS is an
analogous variable for clang targets.

This is useful functionality for selectively disabling the building of clang
targets by default to speed up builds.

In terms of implementation, I just followed the model of LLVM's implementation
of this functionality.

llvm-svn: 275006