[CMake] Add detection for the mold linker in AddLLVM.cmake.

mold says it is compatible with GNU ld and gold linkers:

```
$ mold -v
mold 0.9.5 (compatible with GNU ld and GNU gold)
```

And thus it currently gets detected as Gold.

With the following diff, CMake now correctly reports the linker name, and mold keeps being identified as Gold internally for now.

Reviewed By: ldionne, MaskRay

Differential Revision: https://reviews.llvm.org/D110035