AstGen: implement functions with inferred error sets
This commit also reclaims +2 ZIR instruction tags by moving the
following to `extended`:
* func_var_args
* func_extra
* func_extra_var_args
The following ZIR instruction tag is added:
* func_inferred
2083208f19Andrew Kelley committed on 4/19/2021, 10:03:41 PMparent22015c1