[clangd] Disable ExtractVariable for C

[kadircet]

Summary

Currently extract variable doesn't spell the type explicitly and just
uses an auto instead, which is not available in C.

[sammccall]

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

453

I think "non-C++" would be clearer, and don't mention obj-c at all.
Formally C++ is independent of whether obj-c is enabled, and that's relevant here.

[kadircet]

• Update comment

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

private:
   std::unique_ptr<ExtractionContext> Target;
 };
 REGISTER_TWEAK(ExtractVariable)
 bool ExtractVariable::prepare(const Selection &Inputs) {
   // we don't trigger on empty selections for now
   if (Inputs.SelectionBegin == Inputs.SelectionEnd)
     return false;
   const ASTContext &Ctx = Inputs.AST->getASTContext();
+  // FIXME: Enable non-C++ cases once we start spelling types explicitly instead
+  // of making use of auto.
+  if (!Ctx.getLangOpts().CPlusPlus)
+    return false;
   const SourceManager &SM = Inputs.AST->getSourceManager();
   if (const SelectionTree::Node *N =
           computeExtractedExpr(Inputs.ASTSelection.commonAncestor()))
     Target = std::make_unique<ExtractionContext>(N, SM, Ctx);
   return Target && Target->isExtractable();
 }
 
 Expected<Tweak::Effect> ExtractVariable::apply(const Selection &Inputs) {

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

void f() {
       // do while
       do
         a = [[1]];
       while(a < [[3]]);
     }
   )cpp";
   EXPECT_AVAILABLE(AvailableCases);
 
+  ExtraArgs = {"-xc"};
+  const char *AvailableButC = R"cpp(
+    void foo() {
+      int x = [[1]];
+    })cpp";
+  EXPECT_UNAVAILABLE(AvailableButC);
+  ExtraArgs = {};
+
   const char *NoCrashCases = R"cpp(
     // error-ok: broken code, but shouldn't crash
     template<typename T, typename ...Args>
     struct Test<T, Args...> {
     Test(const T &v) :val[[(^]]) {}
       T val;
     };
   )cpp";