This fix allows using enum values in place of literals.

For example:

```
def is_a(a: Literal["a"]): return None
class MyStr(Enum):
    a = "a"
    b = "b"

is_a(MyStr.a.value)
```

Currently this fails with

```
error: Argument 1 to "is_a" has incompatible type "str"; expected "Literal['a']"
```

The fix itself is simple - the special casing of  final status for enums was being
called too late to have an effect. Moving the function call up solves
the problem.

Fixes python#11919

A few bugs in type operations were exposed by #11962. This fixes
them.

Fix #12051, but other use cases are affected as well.

First, when we erase last known values in an union with multiple
Instance types, make sure that the resulting union doesn't have
duplicate erased types. The duplicate items weren't incorrect as such,
but they could cause overly complex error messages and potentially
slow type checking performance.

Second, make the join of a union type and Any commutative.
Previously the result dependend on the order of the operands,
which was clearly incorrect.

A few bugs in type operations were exposed by #11962. This fixes
them.

Fix #12051, but other use cases are affected as well.

First, when we erase last known values in an union with multiple
Instance types, make sure that the resulting union doesn't have
duplicate erased types. The duplicate items weren't incorrect as such,
but they could cause overly complex error messages and potentially
slow type checking performance.

Second, make the join of a union type and Any commutative.
Previously the result dependend on the order of the operands,
which was clearly incorrect.

#11962 can generate large unions with many Instance types with
last_known_value set. This caused our union simplification algorithm
to be extremely slow, as it hit an O(n**2) code path.

We already had a fast code path for unions of regular literal types. This
generalizes it for unions containing Instance types with last known
values (which behave similarly to literals in a literal type context).

Also fix a union simplification bug that I encountered while writing tests
for this change.

Work on #12408.