[clangd] Skip extra round-trip in parsing args in debug builds. NFC

[sammccall]

Summary

This is a clever cross-cutting sanity test for clang's arg parsing I suppose.
But clangd creates thousands of invocations, ~all with identical trivial
arguments, and problems with these would be caught by clang's tests.
This overhead accounts for 10% of total unittest time!

[ilya-biryukov]

LGTM

clang-tools-extra/clangd/Compiler.cpp

void disableUnsupportedOptions(CompilerInvocation &CI) {
   CI.getFrontendOpts().PluginArgs.clear();
   CI.getFrontendOpts().ProgramAction = frontend::ParseSyntaxOnly;
   CI.getFrontendOpts().ActionName.clear();
 }
 
 std::unique_ptr<CompilerInvocation>
 buildCompilerInvocation(const ParseInputs &Inputs, clang::DiagnosticConsumer &D,
                         std::vector<std::string> *CC1Args) {
-  if (Inputs.CompileCommand.CommandLine.empty())
+  llvm::ArrayRef<std::string> Argv = Inputs.CompileCommand.CommandLine;
+  if (Argv.empty())
     return nullptr;
   std::vector<const char *> ArgStrs;
-  for (const auto &S : Inputs.CompileCommand.CommandLine)
+  ArgStrs.reserve(Argv.size() + 1);
+  // In asserts builds, CompilerInvocation redundantly reads/parses cc1 args as
+  // a sanity test. This is not useful to clangd, and costs 10% of test time.
+  // To avoid mismatches between assert/production builds, disable it always.
+  ArgStrs = {Argv.front().c_str(), "-Xclang", "-no-round-trip-args"};
+  for (const auto &S : Argv.drop_front())
     ArgStrs.push_back(S.c_str());
 
   CreateInvocationOptions CIOpts;
   CIOpts.VFS = Inputs.TFS->view(Inputs.CompileCommand.Directory);
   CIOpts.CC1Args = CC1Args;
   CIOpts.RecoverOnError = true;
   CIOpts.Diags =
       CompilerInstance::createDiagnostics(new DiagnosticOptions, &D, false);