compiler: ensure result of `block_comptime` is comptime-known

To avoid this PR regressing error messages, most of the work here has
gone towards improving error notes for why code was comptime-evaluated.
ZIR `block_comptime` now stores a "comptime reason", the enum for which
is also used by Sema. There are two types in Sema:

* `ComptimeReason` represents the reason we started evaluating something
  at comptime.
* `BlockComptimeReason` represents the reason a given block is evaluated
  at comptime; it's either a `ComptimeReason` with an attached source
  location, or it's because we're in a function which was called at
  comptime (and that function's `Block` should be consulted for the
  "parent" reason).

Every `Block` stores a `?BlockComptimeReason`. The old `is_comptime`
field is replaced with a trivial `isComptime()` method which returns
whether that reason is non-`null`.

Lastly, the handling for `block_comptime` has been simplified. It was
previously going through an unnecessary runtime-handling path; now, it
is a trivial sub block exited through a `break_inline` instruction.

Resolves: #22296

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