For some reason, when unifying a function call parameter with an
argument, subsequent calls to the same function will only accept the
type of the substituted argument.
This affect snippets like:
```
def make1() -> C[Literal[1]]:
return ...
def make2() -> C[Literal[2]]:
return ...
def consume(instance: C[Literal[1, 2]]):
pass
consume(make1())
consume(make2())
```
The last statement will result in a compiler error, as the parameter of
consume is replaced with C[Literal[1]].
We fix this by getting a snapshot before performing unification, and
restoring the snapshot after unification succeeds.
David Mak