Rollup merge of #122487 - GuillaumeGomez:rename-stmtkind-local, r=oli-obk

Rename `StmtKind::Local` variant into `StmtKind::Let`

It comes from this [discussion](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Improve.20naming.20of.20.60ExprKind.3A.3ALet.60.3F).

Starting point was:

> I often end up looking at [ExprKind::Let](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/enum.ExprKind.html#variant.Let) instead of Local because of the name. I think renaming it (both the `ExprKind` variant and the Let struct) to `LetPattern` or LetPat could improve the situation as I'm not sure I'm not the only one encountering this issue.

And then it evolved into:

> It's already `Expr::Let` instead of `StmtKind::Local`. Counterproposal: rename `StmtKind::Local` to `StmtKind::Let`.

The goal here is to clear this confusion.

r? `@oli-obk`

compiler/rustc_mir_build/src/thir/cx/block.rs

@@ -63,7 +63,7 @@ impl<'tcx> Cx<'tcx> {
                         // ignore for purposes of the MIR
                         None
                     }
-                    hir::StmtKind::Local(local) => {
+                    hir::StmtKind::Let(local) => {
                         let remainder_scope = region::Scope {
                             id: block_id,
                             data: region::ScopeData::Remainder(region::FirstStatementIndex::new(