swift234

Fix rdar://11087238, a crash on structs inside funcs.  This adds
a source location to the implicit ExtensionDecl (fixing the crash)
and also allows extensions that aren't at the top level.  Eventually
we'll want to constrain where extensions can exist (for example,
global types should require extensions to be at global scope?) but
we don't need to do that now.


Swift SVN r1398