Rollup merge of #98574 - dingxiangfei2009:let-else-thir, r=oli-obk

Lower let-else in MIR

This MR will switch to lower let-else statements in MIR building instead.

To lower let-else in MIR, we build a mini-switch two branches. One branch leads to the matching case, and the other leads to the `else` block. This arrangement will allow temporary lifetime analysis running as-is so that the temporaries are properly extended according to the same rule applied to regular `let` statements.

cc https://github.com/rust-lang/rust/issues/87335

Fix #98672