Finishes up the "big four" non-type decl kinds. Unfortunately,
indiscriminately dropping members from a class affects the layout
of its vtable. That issue is tracked by rdar://problem/31878396.
Proof-of-concept for the above. This shouldn't be common---renames are
far more likely, and those we can track---but occurs when the
swift_wrapper attribute (the implementation of NS_STRING_ENUM) is
active in Swift 4 but not in Swift 3.
Note that this only checks the canonical interface type of the
declaration, because the non-canonical type may contain references to
the declaration's generic parameters.
The next commits will need access from ModuleFile.cpp.
Also stop pretending we care what kind of error we got back, or that
we might get back random other errors that weren't accounted
for. That's good for crash logs, but not actually interesting here.
(Nearly every place that previously looked for OverrideError will
also accept the upcoming TypeError.)
No effective functionality change.
