[constexpr] Fix ICE when memcpy() is given a pointer to an incomplete array

[constexpr] Fix ICE when memcpy() is given a pointer to an incomplete array

Fix code for constant evaluation of __builtin_memcpy() and
__builtin_memmove() that would attempt to divide by zero when given two
pointers to an incomplete array.

Differential Revision: https://reviews.llvm.org/D51855

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@343761 91177308-0d34-0410-b5e6-96231b3b80d8