[CMake] Add install targets for utilities to LLVM exports if LLVM_INSTALL_UTILS=ON

Summary: D56606 was only appending target names to the `LLVM_EXPORTS`/`LLVM_EXPORTS_BUILDTREE_ONLY` properties. Targets showed up correctly in the build-tree `LLVMExports.cmake`, but they were missing in the installed one (as we found in https://bugs.llvm.org/show_bug.cgi?id=40443), because install did not register them explicitly.

Reviewers: mgorny, smeenai, beanz, gottesmm, dschuff, tstellar, serge-sans-paille

Reviewed By: smeenai

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D57383

llvm-svn: 352869