Tell LLVM about read-only captures

`&Freeze` parameters are not only `readonly` within the function,
but any captures of the pointer can also only be used for reads.
This can now be encoded using the `captures(address, read_provenance)`
attribute.

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -251,6 +251,7 @@ pub(crate) enum AttributeKind {
     Writable = 42,
     DeadOnUnwind = 43,
     DeadOnReturn = 44,
+    CapturesReadOnly = 45,
 }
 
 /// LLVMIntPredicate