swift234

[Mangler] mangle the module where an extension is defined if necessary.

If a method is defined within an extension of a class or struct that is
defined in a different module, we mangle the module where the extension is
defined.

If we define function f in module A, and redefine it again in an extension in
module B, we use different mangling to prevent linking in the wrong
SILFunction.

rdar://18057875


Swift SVN r21488