[clang] Add build and install targets for clang libraries

This is modeled after the existing llvm-libraries target. It's a
convenient way to include all clang libraries in a distribution.

This differs slightly from the llvm-libraries target in that it adds any
library added via add_clang_library, whereas llvm-libraries only
includes targets added via add_llvm_library that didn't use the MODULE
or BUILDTREE_ONLY arguments. add_clang_library doesn't appear to have
any equivalents of those arguments, so the conditions don't apply.

Differential Revision: https://reviews.llvm.org/D58269

llvm-svn: 354141