make @errorCast not pop error return traces

[gooncreeper]

Fixes #20177

This is my first time really going deeper into the compiler, so please
spare me if I made some fatal mistake. I decided to tackle this one
since I can across it while debugging a project of mine, and thought it
would be a good place to get a foot in the water of the compiler
internals.

The actual patch to the bug is one line, which leads to
AstGen.callExpr having the proper value for propagate_error_trace.

Additionally, I made print_zir.writeCall print the
pop_error_return_trace value of function calls to help future
debugging.

As an extra goodie, I also refactored the type of
Zir.Flags.packed_modifier to directly use std.builtin.CallModifier,
removing all the messy casting. I did this since I saw the mess while
tracing down this bug.