CodeGen: correct debug info generation

[compnerd]

Summary

Correct the debug info generation where operators would emit a location CFI directive. The generation of this directive requires an associated lexical block. However, if this statement is emitted into the epilogue, there is no associated lexical block as the operation is synthesized by the compiler. If there is no lexical block currently associated with the emission of the loc directive, take the current debug location as the scope.

Add a second unrelated change to prevent UB in the case that the LexicalBlockStack is empty.

TODO: generate test cases

lib/CodeGen/CGDebugInfo.cpp

       SM.getExpansionLoc(CurLoc) == SM.getExpansionLoc(PrevLoc))
     // New Builder may not be in sync with CGDebugInfo.
     if (!Builder.getCurrentDebugLocation().isUnknown() &&
+        LexicalBlockStack.size() &&
         Builder.getCurrentDebugLocation().getScope(CGM.getLLVMContext()) ==
             LexicalBlockStack.back())
       return;

   // Update last state.
   PrevLoc = CurLoc;
 
-  llvm::MDNode *Scope = LexicalBlockStack.back();
+  llvm::MDNode *Scope =
+      LexicalBlockStack.size()
+          ? LexicalBlockStack.back()
+          : Builder.getCurrentDebugLocation().getScope(CGM.getLLVMContext());
   Builder.SetCurrentDebugLocation(llvm::DebugLoc::get(
       getLineNumber(CurLoc), getColumnNumber(CurLoc, ForceColumnInfo), Scope));
 }