[clangd] Fix off-by-one error in CommandMangler (#160029)

SawInput() is intended to be called for every argument after a `--`, but
it was mistakenly being called for the `--` itself.

Partially fixes clangd/clangd#1850

Author: HighCommander4

clang-tools-extra/clangd/CompileCommands.cpp

@@ -270,7 +270,8 @@ void CommandMangler::operator()(tooling::CompileCommand &Command,
   if (auto *DashDash =
           ArgList.getLastArgNoClaim(driver::options::OPT__DASH_DASH)) {
     auto DashDashIndex = DashDash->getIndex() + 1; // +1 accounts for Cmd[0]
-    for (unsigned I = DashDashIndex; I < Cmd.size(); ++I)
+    // Another +1 so we don't treat the `--` itself as an input.
+    for (unsigned I = DashDashIndex + 1; I < Cmd.size(); ++I)
       SawInput(Cmd[I]);
     Cmd.resize(DashDashIndex);
   }

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

@@ -526,6 +526,16 @@ TEST(CommandMangler, RespectsOriginalSysroot) {
                 Not(HasSubstr(testPath("fake/sysroot"))));
   }
 }
+
+TEST(CommandMangler, StdLatestFlag) {
+  const auto Mangler = CommandMangler::forTests();
+  tooling::CompileCommand Cmd;
+  Cmd.CommandLine = {"clang-cl", "/std:c++latest", "--", "/Users/foo.cc"};
+  Mangler(Cmd, "/Users/foo.cc");
+  // Check that the /std:c++latest flag is not dropped
+  EXPECT_THAT(llvm::join(Cmd.CommandLine, " "), HasSubstr("/std:c++latest"));
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang