Objective-C doesn't consider the use of incomplete types as method

Objective-C doesn't consider the use of incomplete types as method
parameter types to be ill-formed. However, it relies on the
completeness of method parameter types when producing metadata, e.g.,
for a protocol, leading IR generating to crash in such cases.

Since there's no real way to tighten down the semantics of Objective-C
here without breaking existing code, do something safe but lame:
suppress the generation of metadata when this happens.

Fixes <rdar://problem/9123036>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@132171 91177308-0d34-0410-b5e6-96231b3b80d8