Add mir_cast_to_immut_ref function

[RyanGlScott]

The MIR backend has a couple of commands that produce a reference value with the same mutability as something else:

• The mir_static function (see mir_static and mir_static_initializer #1941) produces a reference to a static value. If the static value is immutable, then the reference will be immutable, and if the static value is mutable, then the reference will be mutable.
• The mir_slice_value and mir_slice_range_value functions (see mir_slice_value and mir_slice_range_value #1948) produces a slice reference from an array reference. Similarly, the slice reference will have the same mutability as the array reference.

What is missing from these designs is a way to cast from a mutable reference to an immutable one, which is something that is allowed in Rust. We could envision variants of all the commands above that expect a mutable reference and return an immutable one, but this would result in an explosion of different variants of each command.

A somewhat nicer alternative would be to introduce a standalone mir_cast_to_immut_ref : MIRValue -> MIRValue function that performs this cast. That way, we can consolidate all of the logic needed to do the cast into a single command. In fact, this idea shares some similarities with the existing llvm_cast_pointer command, so perhaps there is a way to design this such that llvm_cast_pointer and mir_cast_to_immut_ref share the same underlying machinery.