Infer Coroutine for unannotated async defs

[97littleleaf11]

Fixes #12776
Add an extra check for unannotated async defs when checking function type. async def foo() is now treated like async def foo() -> Any

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

psycopg (https://github.com/psycopg/psycopg)
+ tests/pool/test_pool_async.py:543: error: All conditional function variants must have identical signatures  [misc]
+ tests/pool/test_pool_async.py:543: note: Original:
+ tests/pool/test_pool_async.py:543: note:     def failed(pool: Any) -> Coroutine[Any, Any, Any]
+ tests/pool/test_pool_async.py:543: note: Redefinition:
+ tests/pool/test_pool_async.py:543: note:     def failed(pool: Any) -> Any

aioredis (https://github.com/aio-libs/aioredis)
+ aioredis/client.py:1697: error: Value of type "Coroutine[Any, Any, Any]" must be used  [unused-coroutine]
+ aioredis/client.py:1697: note: Are you missing an await?
+ aioredis/client.py:4426: error: Return type "Pipeline | Awaitable[Pipeline]" of "execute_command" incompatible with return type "Coroutine[Any, Any, Any]" in supertype "Redis"  [override]

discord.py (https://github.com/Rapptz/discord.py)
+ discord/member.py:199: error: All conditional function variants must have identical signatures  [misc]
+ discord/member.py:199: note: Original:
+ discord/member.py:199: note:     def general(self: Any, *args: Any, **kwargs: Any) -> Coroutine[Any, Any, Any]
+ discord/member.py:199: note: Redefinition:
+ discord/member.py:199: note:     def general(self: Any, *args: Any, **kwargs: Any) -> Any

tornado (https://github.com/tornadoweb/tornado)
+ tornado/test/asyncio_test.py:90: error: Argument 1 to "to_asyncio_future" has incompatible type "Coroutine[Any, Any, Any]"; expected "Future[Any]"  [arg-type]

graphql-core (https://github.com/graphql-python/graphql-core)
+ tests/utils/assert_equal_awaitables_or_values.py:26: error: Incompatible return value type (got "Coroutine[Any, Any, Any]", expected "T")  [return-value]
+ tests/execution/test_stream.py:1699: error: Incompatible types in assignment (expression has type "str", variable has type "Coroutine[Any, Any, Any]")  [assignment]
+ tests/execution/test_stream.py:1765: error: Incompatible types in assignment (expression has type "str", variable has type "Coroutine[Any, Any, Any]")  [assignment]
+ tests/execution/test_abstract.py:61: error: All conditional function variants must have identical signatures  [misc]
+ tests/execution/test_abstract.py:61: note: Original:
+ tests/execution/test_abstract.py:61: note:     def is_type_of(obj: Any, _info: Any) -> Any
+ tests/execution/test_abstract.py:61: note: Redefinition:
+ tests/execution/test_abstract.py:61: note:     def is_type_of(obj: Any, _info: Any) -> Coroutine[Any, Any, Any]
+ tests/execution/test_abstract.py:77: error: All conditional function variants must have identical signatures  [misc]
+ tests/execution/test_abstract.py:77: note: Original:
+ tests/execution/test_abstract.py:77: note:     def type_error(*_args: Any) -> Any
+ tests/execution/test_abstract.py:77: note: Redefinition:
+ tests/execution/test_abstract.py:77: note:     def type_error(*_args: Any) -> Coroutine[Any, Any, Any]