When importing a property as accessor methods (rather than as a
property), we were still importing the type of the accessor methods as
if they were Swift getters and setters of a property, which
(necessarily) homogenizes the types. The homogenization is unnecessary
when importing as accessor methods, because the methods are
independent, so just import the accessor method types as if the
property did not exist. This is particularly useful for maintaining
Swift 3 source compatibility for cases where Swift 4 turns a
getter/setter pair into a null_resettable property.
Fixes rdar://problem/30075571.
Swift side of this new flag. This allows Objective-C framework authors
to replace a pair of methods by properties without breaking source
compatibility. This is especially important for class properties,
which were only introduced last year.
Still to come: importing the accessors even when this flag isn't set,
in order to provide better QoI when migrating from a method interface
to a property interface.
Part of rdar://problem/28455962
