Revert "build: reserve `--color-diagnostics` for lld"

This reverts SVN r330158.

Seems that there was a change to linker flags handling in SVN r316972.
That would alter the behaviour to correct the linker flag handling in
CMake (requiring CMake 3.4.3+).  Since that is already the minimum
version required for LLVM, hard coding the knowledge of the linker is
not required, which is a strictly better solution.

llvm-svn: 330161