[clang-format] Handle return type auto followed by l_paren

[owenpan]

Summary

Fixes https://github.com/llvm/llvm-project/issues/57160.

clang/lib/Format/TokenAnnotator.cpp

else if (Right.is(tok::r_paren) && Right.MatchingParen)
       LeftParen = Right.MatchingParen;
     if (LeftParen && LeftParen->Previous &&
         isKeywordWithCondition(*LeftParen->Previous)) {
       return true;
     }
   }
 
   // trailing return type 'auto': []() -> auto {}, auto foo() -> auto {}
-  if (Left.is(tok::kw_auto) &&
-      Right.isOneOf(TT_LambdaLBrace, TT_FunctionLBrace))
+  if (Left.is(tok::kw_auto) && Right.isOneOf(TT_LambdaLBrace, TT_FunctionLBrace,
+                                             // function return type 'auto'
+                                             TT_FunctionTypeLParen)) {
     return true;
+  }
 
   // auto{x} auto(x)
   if (Left.is(tok::kw_auto) && Right.isOneOf(tok::l_paren, tok::l_brace))
     return false;
 
   // operator co_await(x)
   if (Right.is(tok::l_paren) && Left.is(tok::kw_co_await) && Left.Previous &&
       Left.Previous->is(tok::kw_operator)) {

clang/unittests/Format/FormatTest.cpp

TEST_F(FormatTest, FormatDecayCopy) {
   verifyFormat("auto(x);");
   verifyFormat("auto{x};");
   verifyFormat("new auto{x};");
   verifyFormat("auto{x} = y;");
   verifyFormat("auto(x) = y;"); // actually a declaration, but this is clearly
                                 // the user's own fault
   verifyFormat("integral auto(x) = y;"); // actually a declaration, but this is
                                          // clearly the user's own fault
-  verifyFormat("auto(*p)() = f;");       // actually a declaration; TODO FIXME
+  verifyFormat("auto (*p)() = f;");
 }
 
 TEST_F(FormatTest, Cpp20ModulesSupport) {
   FormatStyle Style = getLLVMStyle();
   Style.AllowShortBlocksOnASingleLine = FormatStyle::SBS_Never;
   Style.AllowShortFunctionsOnASingleLine = FormatStyle::SFS_None;
 
   verifyFormat("export import foo;", Style);