Auto merge of #127670 - compiler-errors:no-type-length-limit, r=jackh726

Gate the type length limit check behind a nightly flag

Effectively disables the type length limit by introducing a `-Zenforce-type-length-limit` which defaults to **`false`**, since making the length limit actually be enforced ended up having a worse fallout than expected. We still keep the code around, but the type length limit attr is now a noop (except for its usage in some diagnostics code?).

r? `@lcnr` -- up to you to decide what team consensus we need here since this reverses an FCP decision.

Reopens #125460 (if we decide to reopen it or keep it closed)
Effectively reverses the decision FCP'd in #125507
Closes #127346

Author: bors