Introduce `Module.ensureFuncBodyAnalyzed` and corresponding `Sema`
function. This mirrors `ensureDeclAnalyzed` except also waits until the
function body has been semantically analyzed, meaning that inferred
error sets will have been populated.
Resolving error sets can now emit a "unable to resolve inferred error
set" error instead of producing an incorrect error set type. Resolving
error sets now calls `ensureFuncBodyAnalyzed`. Closes#11046.
`coerceInMemoryAllowedErrorSets` now does a lot more work to avoid
resolving an inferred error set if possible. Same with
`wrapErrorUnionSet`.
Inferred error set types no longer check the `func` field to determine if
they are equal. That was incorrect because an inline or comptime function
call produces a unique error set which has the same `*Module.Fn` value for
this field. Instead we use the `*Module.Fn.InferredErrorSet` pointers to
test equality of inferred error sets.
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