Avoid calling ParseCommandLineOptions in BackendUtil if possible

[teemperor]

Summary

Calling ParseCommandLineOptions should only be called from main as the
CommandLine setup code isn't thread-safe. As BackendUtil is part of the
generic Clang FrontendAction logic, a process which has several threads executing
Clang FrontendActions will randomly crash in the unsafe setup code.

This patch avoids calling the function unless either the debug-pass option or
limit-float-precision option is set. Without these two options set the
ParseCommandLineOptions call doesn't do anything beside parsing
the command line clang which doesn't set any options.

See also D99652 where LLDB received a workaround for this crash.

[JDevlieghere]

LGTM

clang/lib/CodeGen/BackendUtil.cpp

static void setCommandLineOpts(const CodeGenOptions &CodeGenOpts) {
   if (!CodeGenOpts.DebugPass.empty()) {
     BackendArgs.push_back("-debug-pass");
     BackendArgs.push_back(CodeGenOpts.DebugPass.c_str());
   }
   if (!CodeGenOpts.LimitFloatPrecision.empty()) {
     BackendArgs.push_back("-limit-float-precision");
     BackendArgs.push_back(CodeGenOpts.LimitFloatPrecision.c_str());
   }
+  // Check for the default "clang" invocation that won't set any cl::opt values.
+  // Skip trying to parse the command line invocation to avoid the issues
+  // described below.
+  if (BackendArgs.size() == 1)
+    return;
   BackendArgs.push_back(nullptr);
+  // FIXME: The command line parser below is not thread-safe and shares a global
+  // state, so this call might crash or overwrite the options of another Clang
+  // instance in the same process.
   llvm::cl::ParseCommandLineOptions(BackendArgs.size() - 1,
                                     BackendArgs.data());
 }
 
 void EmitAssemblyHelper::CreateTargetMachine(bool MustCreateTM) {
   // Create the TargetMachine for generating code.
   std::string Error;
   std::string Triple = TheModule->getTargetTriple();