CMake: Add LLVM_UTILS_INSTALL_DIR option

Summary:
This is like the LLVM_TOOLS_INSTALL_DIR option, but for the utils
that are installed when the LLVM_INSTALL_UTILS.  This option
defaults to 'bin' to remain consistent with the current behavior, but
distros may want to install these to libexec/llvm.

Reviewers: beanz

Reviewed By: beanz

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D30655

llvm-svn: 307150