Sema: implement comptime `coerce_result_ptr` and `alloc`

This is progress towards being able to use `builtin.cpu.arch` instead of
the `stage2_arch` workaround. The next blocker is comptime `elemVal`.
e851d89113Andrew Kelley committed on 10/13/2021, 9:16:01 PMparent2a701a9
4 files changedLine totals unavailable