Add FIXME for auditing optional parameters passed to DIBuilder

Author: Zalathar

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1624,6 +1624,9 @@ unsafe extern "C" {
     ) -> &'a Value;
 }
 
+// FIXME(Zalathar): Audit all `Option` parameters, especially in lists, to
+// check that they really are nullable on the C/C++ side. LLVM doesn't appear
+// to actually document which ones are nullable.
 unsafe extern "C" {
     pub(crate) fn LLVMCreateDIBuilder<'ll>(M: &'ll Module) -> *mut DIBuilder<'ll>;
     pub(crate) fn LLVMDisposeDIBuilder<'ll>(Builder: ptr::NonNull<DIBuilder<'ll>>);