This checks that all combinations of optimized & non-optimized, and whole-module
optimization & incremental compilation give the same result, on a module where
this is actually interesting (i.e. has multiple files so the behaviour differs
between the two).
We need to be looking at the linkage of the witness, but mangling the
requirement. Also, getGetter() gets the getter, not the setter (copy-paste
strikes again).
Fixes rdar://problem/40355657.
