[libcxx] [test] Use ptrdiff_t rather than int in splice_after_range.pass.cpp to avoid narrowing from pointer subtraction to int warnings.

Reviewed as https://reviews.llvm.org/D60104

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@357546 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed