[cmake] Create the LLVM_BUILD_UTILS option.

This option is the equivalent option to LLVM_BUILD_TOOLS but for executables
created via add_llvm_utility.

This is a useful tool for improving compile time in situations where LLVM is
used as a library and no testing tools are needed.

It follows the exact same implemention model as LLVM_BUILD_TOOLS.

Since the option is by default set to on, no behavior is changed unless one sets
it from the command line to be false.

llvm-svn: 275007