Auto merge of #50783 - pnkfelix:issue-27282-match-borrows-its-input-take-three, r...

every match arm reads the match's borrowed input

This PR changes the `match` codegen under NLL (and just NLL, at least for now) to make the following adjustments:
 * It adds a `-Z disable-ast-check-for-mutation-in-guard` which, as described, turns off the naive (conservative but also not 100% sound) check for mutation in guards of match arms.
 * We now borrow the match input at the outset and emit a special `ReadForMatch` statement (that, according to the *static* semantics, reads that borrowed match input) at the start of each match arm. The intent here is to catch cases where the match guard mutates the match input, either via an independent borrow or via `ref mut` borrows in that arm's pattern.
 * In order to ensure that `ref mut` borrows do not actually conflict with the emitted `ReadForMatch` statements, I expanded the two-phase-borrow system slightly, and also changed the MIR code gen so that under NLL, when there is a guard on a match arm, then each pattern variable ends up having *three* temporaries associated with it:
   1. The first temporary will hold the substructure being matched; this is what we will move the (substructural) value into *if* the guard succeeds.
   2. The second temporary also corresponds to the same value as the first, but we are just constructing this temporarily for use during the scope of the guard; it is unaliased and its sole referrer is the third temporary.
   3. The third temporary is a reference to the second temporary.
   * (This sounds complicated, I know, but its actually *simpler* than what I was doing before and had checked into the repo, which was to sometimes construct the final value and then take a reference to it before evaluating the guard. See also PR #49870.)

Fix #27282

This also provides a path towards resolving #24535 aka rust-lang/rfcs#1006, at least once the `-Z disable-ast-check-for-mutation-in-guard` is just turned on by default (under NLL, that is. It is not sound under AST-borrowck).
 * But I did not want to make `#![feature(nll)]` imply that as part of this PR; that seemed like too drastic a change to me.