[libcxx] [test] Fix MSVC warning C4244 "conversion from 'X' to 'Y', possible loss...

[libcxx] [test] Fix MSVC warning C4244 "conversion from 'X' to 'Y', possible loss of data", part 6/7.

test/std/algorithms/alg.modifying.operations/alg.random.shuffle/random_shuffle_rand.pass.cpp
(Affects 64-bit architectures.) Include <cstddef> so we can take/return std::ptrdiff_t
(instead of int) in random_shuffle()'s RNG. (C++14 D.12 [depr.alg.random.shuffle]/2 says that
difference_type is used, and we're shuffling a plain array.)

test/std/algorithms/alg.sorting/alg.sort/sort/sort.pass.cpp
test/std/algorithms/alg.sorting/alg.sort/stable.sort/stable_sort.pass.cpp
(Affects 64-bit architectures.) Include <iterator> because we're already using iterator_traits.
Then, store the result of subtracting two RanIts as difference_type instead of long
(which truncates on LLP64 architectures like MSVC x64).

test/std/containers/sequences/forwardlist/forwardlist.ops/splice_after_flist.pass.cpp
test/std/containers/sequences/forwardlist/forwardlist.ops/splice_after_one.pass.cpp
(Affects 64-bit architectures.) Include <cstddef> so we can store the result of
subtracting two pointers as std::ptrdiff_t (instead of int).

test/std/input.output/iostream.format/input.streams/istream.unformatted/ignore_0xff.pass.cpp
(Affects 32-bit architectures.) Sometimes, size_t is too small. That's the case here,
where tellg() returns pos_type (N4606 27.7.2.3 [istream.unformatted]/39). Implementations can
have 64-bit pos_type (to handle large files) even when they have 32-bit size_t.

Fixes D27543.

llvm-svn: 289110