Avoid op= on struct rvalue

[ntrel]

I'm trying to make assigning to a struct rvalue an error, because often the assignment is silently thrown away:
dlang/dmd#21717 (comment)

That breaks some buildkite projects, including this one - so it will probably be a deprecation/error in an edition. This PR avoids breaking this project.

Use opOpAssign directly instead, which is in the spec:

an instance method can still be called on an rvalue struct instance,
even if the method is not const

https://dlang.org/spec/struct.html#member-functions

Alternatively, we could declare an lvalue for the temporary.

[ntrel]

Test failure seems unrelated:

../source/mir/rc/ptr.d(279): Error: returning `createRCWithContext(context._get_value().s, move(context))` escapes a reference to parameter `context`
../source/mir/rc/ptr.d(279):        perhaps annotate the parameter with `return`
../source/mir/rc/ptr.d(425): Error: template instance `mir.rc.ptr.castTo!(S, C)` error instantiating

[ntrel]

Thanks!