stage2: result location types for function call arguments

 * AstGen: restore the param_type ZIR instruction and pass it to the
   expression for function call arguments. This does not solve the
   problem for generic function parameters, but it catches stage2 up to
   stage1 which also does not solve the problem for generic function
   parameters.
   - Most of the enhancements in this commit will still be needed for a
     more sophisticated further improvement to handle generic function
     types.
   - In Sema, handling of `as` coercion recognizes the `var_args_param`
     Type Tag and passes the operand through doing no coercion.
   - That was the last ZIR tag and we are now using all 256 ZIR tags.
 * AstGen: array init and struct init expressions use the anon form even
   when the result location has a type. Prevents the type system
   incorrectly believing, for example, that a tuple is actually an array
   when the result location is a param_type of a function with `anytype`
   parameter.
 * Sema: add missing coercion in `unionInit` to coerce the init to the
   corresponding union field type.
 * `Value.fieldValue` now takes a type and does not take an allocator.

closes #11293

After this commit, stage2 passes all the parser tests.

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