Fix false positive with warning -Wnon-c-typedef-for-linkage

[rtrieu]

Summary

Fix false positives for -Wnon-c-typedef-for-linkage

Implicit methods for structs can confuse the warning, so exclude checking the Decl's that are implicit. Implicit Decl's for lambdas still need to be checked, so skipping all implicit Decl's won't work.

[rsmith]

Looks good, thanks!

clang/lib/Sema/SemaDecl.cpp

if (isa<FriendDecl>(D))
       return {NonCLikeKind::Friend, D->getSourceRange()};
 
     //  -- declare any members other than non-static data members, member
     //     enumerations, or member classes,
     if (isa<StaticAssertDecl>(D) || isa<IndirectFieldDecl>(D) ||
         isa<EnumDecl>(D))
       continue;
     auto *MemberRD = dyn_cast<CXXRecordDecl>(D);
-    if (!MemberRD)
+    if (!MemberRD) {
+      if (D->isImplicit())
+        continue;
       return {NonCLikeKind::OtherMember, D->getSourceRange()};
+    }
 
     //  -- contain a lambda-expression,
     if (MemberRD->isLambda())
       return {NonCLikeKind::Lambda, MemberRD->getSourceRange()};
 
     //  and all member classes shall also satisfy these requirements
     //  (recursively).
     if (MemberRD->isThisDeclarationADefinition()) {

clang/test/SemaCXX/anonymous-struct.cpp

#endif
   } B; // expected-note 0-1{{by this typedef}}
 
   template<typename T> void f() {}
   typedef struct { // expected-error {{unsupported}}
     enum X {};
     int arr[&f<X> ? 1 : 2];
   } C; // expected-note {{by this typedef}}
 }
+
+namespace ImplicitDecls {
+  struct Destructor { ~Destructor() {} };
+  typedef struct {} Empty;
+
+  typedef struct {
+    Destructor x;
+  } A;
+
+  typedef struct {
+    Empty E;
+  } B;
+
+  typedef struct {
+    const Empty E;
+  } C;
+}