assign_coords with mixed DataArray / array args removes coords

I'm not sure if using assign_coords to overwrite the data of coords is the best way to do so, but using mixed args (on current master) turns out to have surprising results:

>>> obj = xr.DataArray(
...     data=[6, 3, 4, 6],
...     coords={"x": list("abcd"), "y": ("x", range(4))},
...     dims="x",
... )
>>> obj
<xarray.DataArray 'obj' (x: 4)>
array([6, 3, 4, 6])
Coordinates:
  * x        (x) <U1 'a' 'b' 'c' 'd'
    y        (x) int64 0 1 2 3
>>> # works as expected
>>> obj.assign_coords(coords={"x": list("efgh"), "y": ("x", [0, 2, 4, 6])})
<xarray.DataArray 'obj' (x: 4)>
array([6, 3, 4, 6])
Coordinates:
  * x        (x) <U1 'e' 'f' 'g' 'h'
    y        (x) int64 0 2 4 6
>>> # works, too (same as .data / .values)
>>> obj.assign_coords(coords={
...     "x": obj.x.copy(data=list("efgh")).variable,
...     "y": ("x", [0, 2, 4, 6]),
... })
<xarray.DataArray 'obj' (x: 4)>
array([6, 3, 4, 6])
Coordinates:
  * x        (x) <U1 'e' 'f' 'g' 'h'
    y        (x) int64 0 2 4 6
>>> # this drops "y"
>>> obj.assign_coords(coords={
...     "x": obj.x.copy(data=list("efgh")),
...     "y": ("x", [0, 2, 4, 6]),
... })
<xarray.DataArray 'obj' (x: 4)>
array([6, 3, 4, 6])
Coordinates:
  * x        (x) <U1 'e' 'f' 'g' 'h'

Passing a DataArray for y, like obj.y * 2 while also changing x (the type does not matter) always results in a MergeError:

>>> obj.assign_coords(x=list("efgh"), y=obj.y * 2)
xarray.core.merge.MergeError: conflicting values for index 'x' on objects to be combined:
first value: Index(['e', 'f', 'g', 'h'], dtype='object', name='x')
second value: Index(['a', 'b', 'c', 'd'], dtype='object', name='x')

I would expect the result to be the same regardless of the type of the new coords.

Start at DataArray.assign_coords and follow the handling of mixed DataArray, Variable, and array coordinate arguments into the merge path. Reproduce the examples from the issue, including the MergeError case, and verify that replacing x preserves y and that equivalent coordinate types produce the same result.