stage2: struct, union, enum, opaque, error sets get better names
This commit takes advantage of the new "NameStrategy" that is exposed
in the ZIR in order to name Decls after their parent when asked to. This
makes the next failing test case pass.
dae22a0a1fAndrew Kelley committed on 5/11/2021, 5:50:00 AMparentb9a099e