[CodeGen][ObjC] Fix assertion failure in EmitARCStoreStrongCall.

The assertion fails because EmitValueForIvarAtOffset doesn't get the
correct type of the ivar when the class the ivar belongs to is
parameterized. This commit fixes the function to compute the ivar's type
based on the type argument provided to the parameterized class.

rdar://problem/32461723

Differential Revision: https://reviews.llvm.org/D33698

llvm-svn: 304449