Validate let expressions

Emit an error if they're found in an invalid position.

crates/syntax/test_data/parser/validation/invalid_let_expr.rs

@@ -0,0 +1,14 @@
+fn foo() {
+    const _: () = let _ = None;
+
+    let _ = if true { (let _ = None) };
+
+    if true && (let _ = None) {
+        (let _ = None);
+        while let _ = None {
+            match None {
+                _ if let _ = None => { let _ = None; }
+            }
+        }
+    }
+}