[clang-tools-extra] [clang-tidy] introduce a must use check (PR #76101)

[Tyler Rockwood via cfe-commits]

================
@@ -54,6 +55,7 @@ class MiscModule : public ClangTidyModule {
     CheckFactories.registerCheck<MisleadingIdentifierCheck>(
         "misc-misleading-identifier");
     CheckFactories.registerCheck<MisplacedConstCheck>("misc-misplaced-const");
+    CheckFactories.registerCheck<MustUseCheck>("misc-must-use");
----------------
rockwotj wrote:

Is there a difference between isReferenced versus the following:

```
decl().bind("var"),         hasAncestor(functionDecl(unless(hasDescendant(declRefExpr(to(decl(equalsBoundNode("var"))))))))
```

Sorry still new to AST matchers

https://github.com/llvm/llvm-project/pull/76101