Rollup merge of #79051 - LeSeulArtichaut:if-let-guard, r=matthewjasper

Implement if-let match guards

Implements rust-lang/rfcs#2294 (tracking issue: #51114).

I probably should do a few more things before this can be merged:
- [x] Add tests (added basic tests, more advanced tests could be done in the future?)
- [x] Add lint for exhaustive if-let guard (comparable to normal if-let statements)
- [x] Fix clippy

However since this is a nightly feature maybe it's fine to land this and do those steps in follow-up PRs.

Thanks a lot `@matthewjasper` ❤️ for helping me with lowering to MIR! Would you be interested in reviewing this?
r? `@ghost` for now

compiler/rustc_hir_pretty/src/lib.rs

@@ -2002,6 +2002,15 @@ impl<'a> State<'a> {
                     self.print_expr(&e);
                     self.s.space();
                 }
+                hir::Guard::IfLet(pat, e) => {
+                    self.word_nbsp("if");
+                    self.word_nbsp("let");
+                    self.print_pat(&pat);
+                    self.s.space();
+                    self.word_space("=");
+                    self.print_expr(&e);
+                    self.s.space();
+                }
             }
         }
         self.word_space("=>");