[analyzer] Make CStringChecker correctly calculate return value of mempcpy

The return value of mempcpy is only correct when the destination type is
one byte in size. This patch casts the argument to a char* so the
calculation is also correct for structs, ints etc.

A patch by Daniel Fahlgren!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219024 91177308-0d34-0410-b5e6-96231b3b80d8