[lld] add context-sensitive PGO options for MachO

Enable support for CSPGO for lld MachO targets.

Since lld MachO does not support `-plugin-opt=`, we need to create the `--cs-profile-generate` and `--cs-profile-path=` options and propagate them in `Darwin.cpp`. These flags are not supported by ld64.

Also outline code into `getLastCSProfileGenerateArg()` to share between `CommonArgs.cpp` and `Darwin.cpp`.

CSPGO is already implemented for ELF (https://reviews.llvm.org/D56675) and COFF (https://reviews.llvm.org/D98763).

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D151589