[Sema] Fix bug in handling of designated initializer.

CheckDesignatedInitializer wasn't taking into account the base classes
when computing the index for the field in the derived class, which
caused the test case to crash during IRGen because of a malformed AST.

rdar://problem/26795040

Differential Revision: https://reviews.llvm.org/D28705

llvm-svn: 292245