Fix list_combine()'s ptype finalization when default is involved

[DavisVaughan]

Closes #2094

Basic idea is that when we see this

list_combine(
  x = list(NA),
  indices = list(1),
  size = 1,
  default = "a"
)

We need the common type across !!!x and default.

We "optimize" the ptype computation in list_combine() as

vec_ptype2(
  vec_ptype_common(x, .arg = x_arg),
  default,
  y_arg = default_arg
)

rather than as

vec_ptype_common(!!!x, default_arg := default)

Because this avoids us having to reallocate a possibly very large list just to slip default in there. It also avoids forcing default_arg. I'm also not entirely sure how we'd use x_arg if we did that.

But in this particular list_combine() example, we get

vec_ptype2(
  vec_ptype_common(x), # this returns a finalized `logical()`
  default, # this is `character()`
  y_arg = default_arg
)
# Error cant combine <logical> and <character>

So we actually need a way to tell vec_ptype_common() that we'd like to opt out of ptype finalization and promise to do it "manually" later on. Now that we've ensured that:

• vec_ptype() and vec_ptype2() never finalize
• vec_ptype_common() always finalizes

I think it makes some sense that we should have an internal way to make vec_ptype_common() act exactly like repeated vec_ptype2() for cases like this one.

[DavisVaughan]

All of this is so we can set this to false

If you look at how ptype_common_with_default() works, it computes the common type of the list of xs using vec_ptype_common(), then needs to incorporate default's type via vec_ptype2() as well.

So we need the result of vec_ptype_common() to not be finalized one the way out, we will do it manually after calling vec_ptype2().

An example of this came up in revdeps with case_when() #2094

I consider this an internal "performance optimization". An alternative way to do this is to combine the list of xs with default and default_arg as something like list2(!!!xs, default_arg := default) but this is way too expensive for something like list_combine(). The point of this helper is to avoid requiring that.