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/src/hir.rs

@@ -1160,6 +1160,7 @@ pub struct Arm<'hir> {
 #[derive(Debug, HashStable_Generic)]
 pub enum Guard<'hir> {
     If(&'hir Expr<'hir>),
+    IfLet(&'hir Pat<'hir>, &'hir Expr<'hir>),
 }
 
 #[derive(Debug, HashStable_Generic)]
@@ -1721,6 +1722,8 @@ pub enum MatchSource {
     IfDesugar { contains_else_clause: bool },
     /// An `if let _ = _ { .. }` (optionally with `else { .. }`).
     IfLetDesugar { contains_else_clause: bool },
+    /// An `if let _ = _ => { .. }` match guard.
+    IfLetGuardDesugar,
     /// A `while _ { .. }` (which was desugared to a `loop { match _ { .. } }`).
     WhileDesugar,
     /// A `while let _ = _ { .. }` (which was desugared to a
@@ -1739,7 +1742,7 @@ impl MatchSource {
         use MatchSource::*;
         match self {
             Normal => "match",
-            IfDesugar { .. } | IfLetDesugar { .. } => "if",
+            IfDesugar { .. } | IfLetDesugar { .. } | IfLetGuardDesugar => "if",
             WhileDesugar | WhileLetDesugar => "while",
             ForLoopDesugar => "for",
             TryDesugar => "?",