This enables additional tests for the ClangImporter. This found a
missing piece in the `-enable-objc-interop` work that was done
previously. Address that and enable the tests. There are now the
following failing tests on Linux:
* sdk - depends on Foundation (not hermetic, see SR-7572)
* mixed-nsuinteger - depends on Foundation (not hermetic, see SR-7572)
* import-mixed-with-header-twice - requires apple/swift PR#16022
* can_import_objc_idempotent - requires apple/swift PR#16022
* objc_protocol_renaming - requires apple/swift PR#16022
Previously, the importer queued up conformances to complete once it
was done importing the current batch of declarations. However, if
there was a serialized Swift module that extended an imported type to
add a conformance in exactly the wrong way, the importer could end up
asking for that conformance later---even before the reference to the
imported type was resolved. This led to a crash in the deserializer
"while reading conformance for type X".
Instead of this "pending actions" queue, we can just use the
mechanisms already in place for lazily loading conformances. That way
they'll get filled out on demand, which is better all around anyway.
This does mean putting the requirement signature into the "lazy" part
of the conformance, though.
This does as a side effect mean that /all/ of the witnesses for the
imported conformance may be opaque---that is, they will never be
devirtualized to a particular implementation. However, they previously
would have referred to methods implemented in Objective-C anyway,
which are always dispatched with objc_msgSend. So this should have no
practical effect.
rdar://problem/32346184
