[clangd] Disable ExtractFunction for C

[kadircet]

Summary

This tweak uses constructs like auto and refs, which are not available
in C.

[hokein]

This tweak uses constructs like auto and refs, which are not available in C.

I didn't find this tweak is using auto. I think we need to do the same thing for extract variable tweak as well (only for C++11).

clang-tools-extra/clangd/refactor/tweaks/ExtractFunction.cpp

tooling::Replacement createFunctionDefinition(const NewFunction &ExtractedFunc,
   std::string FunctionDef = ExtractedFunc.renderDefinition(SM);
   return tooling::Replacement(SM, ExtractedFunc.InsertionPoint, 0, FunctionDef);
 }
 
 bool ExtractFunction::prepare(const Selection &Inputs) {
   const Node *CommonAnc = Inputs.ASTSelection.commonAncestor();
   const SourceManager &SM = Inputs.AST->getSourceManager();
   const LangOptions &LangOpts = Inputs.AST->getLangOpts();
+  if (!LangOpts.CPlusPlus)
+    return false;
   if (auto MaybeExtZone = findExtractionZone(CommonAnc, SM, LangOpts)) {
     ExtZone = std::move(*MaybeExtZone);
     return true;
   }
   return false;
 }
 
 Expected<Tweak::Effect> ExtractFunction::apply(const Selection &Inputs) {

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

TEST_F(ExtractFunctionTest, FunctionTest) {
   // initializer/condition.
   EXPECT_THAT(apply(" for([[int i = 0;]];);"), HasSubstr("extracted"));
   // Don't extract because needs hoisting.
   EXPECT_THAT(apply(" [[int a = 5;]] a++; "), StartsWith("fail"));
   // Extract certain return
   EXPECT_THAT(apply(" if(true) [[{ return; }]] "), HasSubstr("extracted"));
   // Don't extract uncertain return
   EXPECT_THAT(apply(" if(true) [[if (false) return;]] "), StartsWith("fail"));
+
+  FileName = "a.c";
+  EXPECT_THAT(apply(" for([[int i = 0;]];);"), HasSubstr("unavailable"));
 }
 
 TEST_F(ExtractFunctionTest, FileTest) {
   // Check all parameters are in order
   std::string ParameterCheckInput = R"cpp(
 struct Foo {
   int x;
 };