[libcxx] [test] Improve MSVC portability.

test/support/msvc_stdlib_force_include.hpp
When testing MSVC's STL with C1XX, simulate a couple more compiler feature-test macros.

When testing MSVC's STL, simulate a few library feature-test macros.

test/std/atomics/atomics.lockfree/isalwayslockfree.pass.cpp
The vector_size attribute is a non-Standard extension that's supported by Clang and GCC,
but not C1XX. Therefore, guard this with `__has_attribute(vector_size)`.

Additionally, while these tests pass when MSVC's STL is compiled with Clang,
I don't consider this to be a supported scenario for our library,
so also guard this with defined(_LIBCPP_VERSION).

test/std/utilities/function.objects/func.not_fn/not_fn.pass.cpp
N4713 23.14.10 [func.not_fn]/1 depicts only `call_wrapper(call_wrapper&&) = default;`
and `call_wrapper(const call_wrapper&) = default;`. According to
15.8.2 [class.copy.assign]/2 and /4, this makes call_wrapper non-assignable.
Therefore, guard the assignability tests as libc++ specific.

Add a (void) cast to tolerate not_fn() being marked as nodiscard.

Fixes D41213.

llvm-svn: 322144

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