Adds three refactorings intended to help users migrate their existing
code to use the new async language features:
1. Convert call to use async alternative
2. Convert function to async
3. Add async alternative function
A function is considered to have an async alternative if it has a void
return type and has a void returning closure as its last parameter. A
method to explicitly mark functions as having an async alternative may
be added to make this more accurate in the future (required for eg.
a warning about a call to the non-async version of a function in an
async context).
(1) converts a call to use the new `await` async language syntax. If the
async alternative throws, it will also add `try`. The closure itself is
hoisted out of the call, see the comments on
`AsyncConversionStringBuilder` for specifics.
(2) converts a whole function to `async`, using (1) to convert any calls
in the function to their async alternatives. (3) is similar to (2), but
instead *adds* a function and replaces calls to its
completion/handler/callback closure parameter with `return` or `throws`.
Resolves rdar://68254700
