CMake: Add a new configuration option LLVM_BUILD_CLANG that's

available when Clang is found within the LLVM tree. If enabled (the
default), Clang will be built as part of LLVM. If disabled, Clang will
be skipped... and can be built by configuring a separate object
directory just for Clang. This helps break up the monolithic
LLVM+Clang project that many Clang developers use, improving
build/load times.

llvm-svn: 135218