Always transform for-await in async functions.

[nicolo-ribaudo]

Q	A
Fixed Issues?	#4969
Patch: Bug Fix?	yes
Major: Breaking Change?	yes?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR
Any Dependency Changes?
License	MIT

for-await was transformed in @babel/helper-remap-async-to-generator, which was called
by @babel/plugin-transform-async-to-generator and
@babel/plugin-proposal-async-generator-functions. This prevented for-await statements
in async functions to be transpiled if the transform-async-to-generator plugin was't
enabled.

This might be considered a breaking change, in case someone used for-await in async functions using just the async functions transform.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/6152/

[mattapperson]

@nicolo-ribaudo bump on this?

[nicolo-ribaudo]

Waiting for reviews.
(And I have to fix the merge conflict, but I don't have time right now)

[mattapperson]

@nicolo-ribaudo anything I can do to help?

[nicolo-ribaudo]

Moved to #7446