swift234

Use extension mangling for all members of cross-module extensions.

If an entity is declared in an extension, and the extension is declared in a
different module from the type it extends, we include the extension's module
name in the mangling in addition to the extended type. We were already doing
this for functions and subscripts because of rdar://problem/18057875, but
this extends it to all declarations.

This is necessary to handle
(1) debugging private members of cross-module extensions
(2) the same member being declared in two different modules, each of which
    extends a type in a third module

rdar://problem/14884749

Swift SVN r21846