Monomorphise `#[repr(transparent)]` parameterized ADTs before turning
them into an Itanium mangled String.
`#[repr(transparent)]` ADTs currently use the single field to represent
them in their CFI type ID to ensure that they are compatible. However,
if that type involves a type parameter instantiated at the ADT level, as
in `ManuallyDrop`, this will currently ICE as the `Parameter` type
cannot be mangled. Since this happens at lowering time, it should always
be concrete after substitution.
Fixes#106230
fb2c27d73f