[SIL] Move from SILFunction to SILFunctionType flag for async.

[nate-chandler]

Previously, the flag had been applied to a SILFunction. That was incorrect because the bit was not part of the type of a function, which meant that only function declarations but not values could be annotated.

[nate-chandler]

@swift-ci please test

[varungandhi-apple]

Could you please move this flag onto SILExtInfoBuilder instead of being on SILFunctionType? ASTExtInfoBuilder has a similar isAsync bit. That will make the changes "just fall out" instead of having to update call-sites for the different constructors and the Profile function later #33559.

(I'm happy to implement that change if you're busy with other stuff.)