CMake: enable installing utils

Added a new boolean CMake flag, LLVM_INSTALL_UTILS. When set,
the 'install' target will include in the bin directory the
utils binaries - e.g. FileCheck. This mirrors the autoconfig
behavior.

Test Plan:
Locally verified that utils binaries are copied when flag is set,
and not copied when flag is not set.

Reviewers: jfb, dschuff, beanz

Reviewed By: beanz

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8587

Patch by Mircea Trofin

llvm-svn: 233385