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

src/codegen/c/Type.zig

@@ -1468,7 +1468,6 @@ pub const Pool = struct {
             .bool_false,
             .empty_struct,
             .generic_poison,
-            .var_args_param_type,
             .none,
             => unreachable,
 

src/codegen/llvm.zig

@@ -3235,7 +3235,6 @@ pub const Object = struct {
             .bool_false,
             .empty_struct,
             .generic_poison,
-            .var_args_param_type,
             .none,
             => unreachable,
             else => switch (ip.indexToKey(t.toIntern())) {