[cxxabi] Get ptrdiff_t from the CodeGenModule instead of caching it

As the comment says, it's a little silly to cache it in the ABI code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177738 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed