Add let_underscore_must_use lint.

Similar to `let_underscore_drop`, this lint checks for statements similar
to `let _ = foo`, where `foo` is an expression marked `must_use`.

compiler/rustc_lint/src/lib.rs

@@ -317,7 +317,12 @@ fn register_builtins(store: &mut LintStore, no_interleave_lints: bool) {
         REDUNDANT_SEMICOLONS
     );
 
-    add_lint_group!("let_underscore", LET_UNDERSCORE_DROP, LET_UNDERSCORE_LOCK);
+    add_lint_group!(
+        "let_underscore",
+        LET_UNDERSCORE_DROP,
+        LET_UNDERSCORE_LOCK,
+        LET_UNDERSCORE_MUST_USE
+    );
 
     add_lint_group!(
         "rust_2018_idioms",