Drop "readonly" LLVM attribute from references to opaque C++ types

[dtolnay]

@capickett noticed that passing a cxx-generated opaque C++ type by reference gets annotated with an unwanted, incorrect readonly attribute. We have observed miscompiles that arise from this in real-world code when using cross-language ThinLTO.

According to https://llvm.org/docs/LangRef.html:

readonly

This attribute indicates that the function does not write through this pointer argument, even though it may write to the memory that the pointer points to.

If a function writes to a readonly pointer argument, the behavior is undefined.

For the following repro:

#[no_mangle]
pub fn repro(opaque: &cxx::private::Opaque) {
    extern "C" {
        fn do_repro(opaque: *const cxx::private::Opaque);
    }
    unsafe { do_repro(opaque) }
}

this is the LLVM IR generated before this PR:

; Function Attrs: nounwind nonlazybind uwtable
define void @repro(ptr noalias noundef nonnull readonly align 1 %opaque) unnamed_addr #0 {
start:
  tail call void @do_repro(ptr noundef nonnull %opaque) #1
  ret void
}

and after:

; Function Attrs: nounwind nonlazybind uwtable
define void @repro(ptr noundef nonnull align 1 %opaque) unnamed_addr #0 {
start:
  tail call void @do_repro(ptr noundef nonnull %opaque) #1
  ret void
}

< define void @repro(ptr noalias noundef nonnull readonly align 1 %opaque) unnamed_addr #0 {
---
> define void @repro(ptr noundef nonnull align 1 %opaque) unnamed_addr #0 {