Immutables converted to immediates

[Iain Buclaw]

On Saturday, 1 July 2023 at 20:52:14 UTC, Iain Buclaw wrote:
> I guess there's scope for applying read-only semantics to both 
> const and immutable locals/parameters.  For static data, only 
> immutables that aren't set in a ctor can be read-only.

Let's see how well 
[this](https://github.com/gcc-mirror/gcc/commit/61b1c562f8c703bff045e91257120e42b7fae523) goes...