[libcxx] [test] D27269: Fix MSVC x64 warning C4267 "conversion from 'size_t'...

[libcxx] [test] D27269: Fix MSVC x64 warning C4267 "conversion from 'size_t' to 'int' [or 'unsigned int'], possible loss of data", part 3/4.

test/std/containers/sequences/vector.bool/copy.pass.cpp
test/std/containers/sequences/vector.bool/copy_alloc.pass.cpp
test/std/containers/sequences/vector/vector.cons/copy.pass.cpp
test/std/containers/sequences/vector/vector.cons/copy_alloc.pass.cpp
Change "unsigned s = x.size();" to "typename C::size_type s = x.size();"
because that's what it returns.

test/std/strings/basic.string/string.cons/pointer_alloc.pass.cpp
Include <cstddef>, then change "unsigned n = T::length(s);"
to "std::size_t n = T::length(s);" because that's what char_traits returns.

test/std/strings/basic.string/string.cons/substr.pass.cpp
Change unsigned to typename S::size_type because that's what str.size() returns.

test/std/utilities/template.bitset/bitset.cons/ull_ctor.pass.cpp
This was needlessly truncating std::size_t to unsigned.
It's being used to compare and initialize std::size_t.

llvm-svn: 288753