[CMake] Export utility targets to the build/install tree depending on LLVM_BUILD/INSTALL_UTILS

Summary:
Allow external projects to import test-related targets like FileCheck, count, not etc. and query binary paths, properties, etc.
This would be useful for LLDB, because it reduces the difference between in-tree vs. standalone builds and simplifies CMake logic.

Reviewers: chapuni, gottesmm, beanz

Reviewed By: beanz

Subscribers: mgorny, lldb-commits, llvm-commits, #lldb

Differential Revision: https://reviews.llvm.org/D56606

llvm-svn: 350959