[clangd] Show argument names for function pointer struct fields (#69011)

Show argument names in signature help when calling a function pointer
struct field.

Author: Qwinci

clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp

@@ -1462,6 +1462,23 @@ TEST(SignatureHelpTest, FunctionPointers) {
     typedef void (__stdcall *fn)(int x, int y);
     fn foo;
     int main() { foo(^); }
+  )cpp",
+      // Field of function pointer type
+      R"cpp(
+    struct S {
+      void (*foo)(int x, int y);
+    };
+    S s;
+    int main() { s.foo(^); }
+  )cpp",
+      // Field of function pointer typedef type
+      R"cpp(
+    typedef void (*fn)(int x, int y);
+    struct S {
+      fn foo;
+    };
+    S s;
+    int main() { s.foo(^); }
   )cpp"};
   for (auto Test : Tests)
     EXPECT_THAT(signatures(Test).signatures,

clang/lib/Sema/SemaCodeComplete.cpp

@@ -6137,6 +6137,7 @@ ProduceSignatureHelp(Sema &SemaRef, MutableArrayRef<ResultCandidate> Candidates,
 // so that we can recover argument names from it.
 static FunctionProtoTypeLoc GetPrototypeLoc(Expr *Fn) {
   TypeLoc Target;
+
   if (const auto *T = Fn->getType().getTypePtr()->getAs<TypedefType>()) {
     Target = T->getDecl()->getTypeSourceInfo()->getTypeLoc();
 
@@ -6145,6 +6146,11 @@ static FunctionProtoTypeLoc GetPrototypeLoc(Expr *Fn) {
     if (const auto *const VD = dyn_cast<VarDecl>(D)) {
       Target = VD->getTypeSourceInfo()->getTypeLoc();
     }
+  } else if (const auto *ME = dyn_cast<MemberExpr>(Fn)) {
+    const auto *MD = ME->getMemberDecl();
+    if (const auto *FD = dyn_cast<FieldDecl>(MD)) {
+      Target = FD->getTypeSourceInfo()->getTypeLoc();
+    }
   }
 
   if (!Target)