proposal: run stage2 astgen when using stage1

[g-w1]

Stage2 has some desirable features that are missing in stage1 in astgen (and I plan to add more like unused variables). This proposal is to run astgen unconditionally on any zig file that is being compiled, c++ or zig implementation.
Pros:
catch more errors earlier
Cons:
a tiny bit slower

[mikdusan]

What happens if astgen produces errors that are redundant to those produced by stage1?

[g-w1]

Astgen will run first, and if there are any errors, stage1 will abort. This can be hooked into stage1's @import command trivially I think.

[g-w1]

This is implemented by #9191

[andrewrk]

Ah thanks, I forgot you filed this.

[andrewrk]

Uh it's not merged yet tho

[andrewrk]

Landed in 31c49ad.