stage2: implement `@intToError` with safety

This commit introduces a new AIR instruction `cmp_lt_errors_len`. It's
specific to this use case for two reasons:

 * The total number of errors is not stable during semantic analysis; it
   can only be reliably checked when flush() is called. So the backend
   that is lowering the instruction must emit a relocation of some kind
   and then populate it during flush().
 * The fewer AIR instructions in memory, the better for compiler
   performance, so we squish complex meanings into AIR tags without
   hesitation.

The instruction is implemented only in the LLVM backend so far. It does
this by creating a simple function which is gutted and re-populated
with each flush().

AstGen now uses ResultLoc.coerced_ty for `@intToError` and Sema does the
coercion.

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