GenericAlias in type expressions

We recognize if a types.GenericAlias instance is created by specializing a generic class. We don't explicitly mention it in our type display, but list[int] in the example below is a GenericAlias instance at runtime:

Numbers = list[int]

# At runtime, `Numbers` is an instance of `types.GenericAlias`. Showing
# this as `list[int]` is more helpful, though:
reveal_type(Numbers)  # revealed: <class 'list[int]'>

def _(numbers: Numbers) -> None:
    reveal_type(numbers)  # revealed: list[int]

It is also valid to create GenericAlias instances manually:

from types import GenericAlias

Strings = GenericAlias(list, (str,))

reveal_type(Strings)  # revealed: GenericAlias

However, using such a GenericAlias instance in a type expression is currently not supported:

# error: [invalid-type-form] "Variable of type `GenericAlias` is not allowed in a parameter annotation"
def _(strings: Strings) -> None:
    reveal_type(strings)  # revealed: Unknown