InternPool: eliminate var_args_param_type

This was a "fake" type used to handle C varargs parameters, much like
generic poison. In fact, it is treated identically to generic poison in
all cases other than one (the final coercion of a call argument), which
is trivially special-cased. Thus, it makes sense to remove this special
tag and instead use `generic_poison_type` in its place. This fixes
several bugs in Sema related to missing handling of this tag.

Resolves: #19781

lib/std/zig/Zir.zig

@@ -2194,9 +2194,6 @@ pub const Inst = struct {
         empty_struct,
         generic_poison,
 
-        /// This tag is here to match Air and InternPool, however it is unused
-        /// for ZIR purposes.
-        var_args_param_type = std.math.maxInt(u32) - 1,
         /// This Ref does not correspond to any ZIR instruction or constant
         /// value and may instead be used as a sentinel to indicate null.
         none = std.math.maxInt(u32),