[clang-tidy] Handle bitfields in cppcoreguidelines-pro-type-member-init if using C++2a

[malcolm.parsons]

Summary

C++2a allows bitfields to have default member initializers.
Add support for this to clang-tidy's cppcoreguidelines-pro-type-member-init check.

[aaron.ballman]

LGTM!

clang-tidy/cppcoreguidelines/ProTypeMemberInitCheck.cpp

void ProTypeMemberInitCheck::checkMissingMemberInitializer(
   // as initializing more than one union member is an error.
   SmallPtrSet<const FieldDecl *, 16> FieldsToFix;
   SmallPtrSet<const RecordDecl *, 4> UnionsSeen;
   forEachField(ClassDecl, OrderedFields, [&](const FieldDecl *F) {
     if (!FieldsToInit.count(F))
       return;
     // Don't suggest fixes for enums because we don't know a good default.
     // Don't suggest fixes for bitfields because in-class initialization is not
-    // possible.
-    if (F->getType()->isEnumeralType() || F->isBitField())
+    // possible until C++2a.
+    if (F->getType()->isEnumeralType() ||
+        (!getLangOpts().CPlusPlus2a && F->isBitField()))
       return;
     if (!F->getParent()->isUnion() || UnionsSeen.insert(F->getParent()).second)
       FieldsToFix.insert(F);
   });
   if (FieldsToFix.empty())
     return;
 
   // Use in-class initialization if possible.

test/clang-tidy/cppcoreguidelines-pro-type-member-init-cxx2a.cpp

+// RUN: %check_clang_tidy %s cppcoreguidelines-pro-type-member-init %t -- -- -std=c++2a -fno-delayed-template-parsing
+
+struct PositiveBitfieldMember {
+  PositiveBitfieldMember() {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: constructor does not initialize these fields: F
+  unsigned F : 5;
+  // CHECK-FIXES: unsigned F : 5{};
+};
+
+struct NegativeUnnamedBitfieldMember {
+  NegativeUnnamedBitfieldMember() {}
+  unsigned : 5;
+};
+
+struct NegativeInitializedBitfieldMembers {
+  NegativeInitializedBitfieldMembers() : F(3) { G = 2; }
+  unsigned F : 5;
+  unsigned G : 5;
+};

test/clang-tidy/cppcoreguidelines-pro-type-member-init.cpp

class B : public NoCrash {
     template <typename U> B(U u) {}
   };
 };
 
 struct PositiveBitfieldMember {
   PositiveBitfieldMember() {}
   // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: constructor does not initialize these fields: F
   unsigned F : 5;
+  // CHECK-FIXES-NOT: unsigned F : 5{};
 };
 
 struct NegativeUnnamedBitfieldMember {
   NegativeUnnamedBitfieldMember() {}
   unsigned : 5;
 };
 
 struct NegativeInitializedBitfieldMembers {