[clang-tools-extra] Add check 'modernize-use-enum-class' (PR #138282)

[Baranov Victor via cfe-commits]

================
@@ -0,0 +1,34 @@
+//===--- UseEnumClassCheck.cpp - clang-tidy -------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "UseEnumClassCheck.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::modernize {
+
+void UseEnumClassCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(
+      traverse(TK_AsIs,
+               enumDecl(unless(isScoped()), unless(hasParent(recordDecl()))))
+          .bind("unscoped_enum"),
+      this);
+}
+
+void UseEnumClassCheck::check(const MatchFinder::MatchResult &Result) {
+  const auto *UnscopedEnum = Result.Nodes.getNodeAs<EnumDecl>("unscoped_enum");
+
+  diag(UnscopedEnum->getLocation(),
+       "enum %0 is unscoped, use enum class instead")
+      << UnscopedEnum;
+  diag(UnscopedEnum->getLocation(), "insert 'class'", DiagnosticIDs::Note)
+      << FixItHint::CreateInsertion(UnscopedEnum->getLocation(), "class ");
----------------
vbvictor wrote:

If we want to support fix-its I think we should at least try to convert all previous usages of enum values to use `::`.
E.g.
```cpp
enum A {
    a
};

int main() {
    A a_c = a;
}
```
Transformed to
```cpp
enum class A { // new 'class' fixit!
    a
};

int main() {
    A a_c = A::a; // new 'A::' fixit!
}
```

https://github.com/llvm/llvm-project/pull/138282