[CMake] Remove custom ccache CMake logic

CMake supports CMAKE_CXX_COMPILER_LAUNCHER since CMake 3.4
so this custom CMake logic we had in LLVM can now be removed.

The only downside with this is that we can't set ccache
options from LLVM CMake, but it's arguable that this doesn't
belong in LLVM but should be done in a script calling the
build.

This was discussed in the forums here:

https://discourse.llvm.org/t/tips-for-incremental-building/67289/4?u=tobiashieta

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D143468