[CMake] Replace clang binary if using clang-bolt target

Instead of creating an extra clang-bolt binary and clang++-bolt symlink,
replace the original clang binary with BOLT-optimized one.

This fixes the issue with installing optimized binary as `install-clang`
target now copies the new version.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D139454