Sema: dummy implementation of `@errorReturnTrace`

Also update std/build.zig to use stage2 function pointer semantics.

This gets us a little bit closer to `zig build` working, although it is
now hitting a new crash in the compiler.
0fb005d1d0Andrew Kelley committed on 3/22/2022, 2:08:30 AMparentbe579d4
3 files changedLine totals unavailable