Avoid calling ParseCommandLineOptions in BackendUtil if possible

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.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D99740

Author: Teemperor

clang/lib/CodeGen/BackendUtil.cpp

@@ -871,7 +871,15 @@ static void setCommandLineOpts(const CodeGenOptions &CodeGenOpts) {
     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());
 }