Sema: remove redundant comptime-known initializer tracking

This logic predates certain Sema enhancements whose behavior it
essentially tries to emulate in one specific case in a problematic way.
In particular, this logic handled initializing comptime-known `const`s
through RLS, which was reworked a few years back in 644041b to not rely
on this logic, and catching runtime fields in comptime-only
initializers, which has since been *correctly* fixed with better checks
in `Sema.storePtr2`. That made the highly complex logic in
`validateStructInit`, `validateUnionInit`, and `zirValidatePtrArrayInit`
entirely redundant. Worse, it was also causing some tracked bugs, as
well as a bug which I have identified and fixed in this PR (a
corresponding behavior test is added).

This commit simplifies union initialization by bringing the runtime
logic more in line with the comptime logic: the tag is now always
populated by `Sema.unionFieldPtr` based on `initializing`, where this
previously happened only in the comptime case (with `validateUnionInit`
instead handling it in the runtime case). Notably, this means that
backends are now able to consider getting a pointer to an inactive union
field as Illegal Behavior, because the `set_union_tag` instruction now
appears *before* the `struct_field_ptr` instruction as you would
probably expect it to.

Resolves: #24520
Resolves: #24595

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