[libcxx] [test] D27025: Fix MSVC warning C4389 "signed/unsigned mismatch", part 12/12.

Various changes:

test/std/algorithms/alg.sorting/alg.merge/inplace_merge.pass.cpp
This is comparing value_type to unsigned. value_type is sometimes int and sometimes struct S (implicitly constructible from int).
static_cast<value_type>(unsigned) silences the warning and doesn't do anything bad (as the values in question are small).

test/std/algorithms/alg.sorting/alg.nth.element/nth_element_comp.pass.cpp
This is comparing an int remote-element to size_t. The values in question are small and non-negative,
so either type is fine. I think that converting int to size_t is marginally better here than the reverse.

test/std/containers/sequences/deque/deque.cons/size.pass.cpp
DefaultOnly::count is int (and non-negative). When comparing to unsigned, use static_cast<unsigned>.

test/std/strings/basic.string/string.access/index.pass.cpp
We're comparing char to '0' through '9', but formed with the type size_t. Add static_cast<char>.

test/std/utilities/template.bitset/bitset.cons/ull_ctor.pass.cpp
Include <cstddef> for pedantic correctness (this test was already mentioning std::size_t).

"v[i] == (i & 1)" was comparing bool to size_t. Saying "v[i] == ((i & 1) != 0)" smashes the RHS to bool.

llvm-svn: 288749