[clangd] Respect `--sysroot` argument if it is set

[dgoldman]

Summary

• Since --sysroot is a superset of the -isysroot argument, we shouldn't add the -isysroot if we detect a --sysroot flag.

[merge_guards_bot]

Unit tests: pass. 61323 tests passed, 0 failed and 736 were skipped.

clang-tidy: pass.

clang-format: pass.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

[sammccall]

Thanks!

I wonder what accident of history led to the different number of dashes between the flags, weird.

clang-tools-extra/clangd/CompileCommands.cpp

void CommandMangler::adjust(std::vector<std::string> &Cmd) const {
   // not support plugins currently. Therefore it breaks if
   // compiler tries to load plugins.
   Cmd = tooling::getStripPluginsAdjuster()(Cmd, "");
   Cmd = tooling::getClangSyntaxOnlyAdjuster()(Cmd, "");
 
   if (ResourceDir && !Has("-resource-dir"))
     Cmd.push_back(("-resource-dir=" + *ResourceDir));
 
-  if (Sysroot && !Has("-isysroot")) {
+  // Don't set `-isysroot` if it is already set or if `--sysroot` is set.
+  // `--sysroot` is a superset of the `-isysroot` argument.
+  if (Sysroot && !Has("-isysroot") && !Has("--sysroot")) {
     Cmd.push_back("-isysroot");
     Cmd.push_back(*Sysroot);
   }
 
   // If the driver is a generic name like "g++" with no path, add a clang path.
   // This makes it easier for us to find the standard libraries on mac.
   if (ClangPath && llvm::sys::path::is_absolute(*ClangPath) && !Cmd.empty()) {
     std::string &Driver = Cmd.front();