c: When checking on validity of sizeof passed as size of
argument to be memset, check for its type to be complete 
before calling Context.getTypeSize(PointeeTy) to prevent 
crash. // rdar://13081751.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173872 91177308-0d34-0410-b5e6-96231b3b80d8