This is all of the expected 0.14.0 progress on #21530, which can now be
postponed once this commit is merged.
This required rewriting the (un)wrap operations since the original
implementations were extremely buggy.
Also adds an easy way to retrigger Sema OPV bugs so that I don't have to
keep updating #22419 all the time.
The compiler actually doesn't need any functional changes for this: Sema
does reification based on the tag indices of `std.builtin.Type` already!
So, no zig1.wasm update is necessary.
This change is necessary to disallow name clashes between fields and
decls on a type, which is a prerequisite of #9938.
