Fixed incorrect header to atomic_helpers.h in atomic_wait.pass.cpp

Modifications based on @ldionne's recommendations

Rebased on main

Changes based on recommendations.

LGTM!

In D103478#2797854, @tcanens wrote:

In D103478#2794869, @Quuxplusone wrote:

I was looking at how to implement compare_three_way and found out that it needs to be present in both <compare> and <functional> according to the standard draft ( <function> synopsis, <compare> synopsis).

Sounds like someone should file an LWG issue. I'm not aware of any other library facilities that are listed multiple times in different places in the Standard. This might be a cut-and-paste error.

This is the new way to say "this thing is provided if you include either header": https://github.com/cplusplus/draft/issues/3683

It's very much intentional that compare_three_way is available if you include either <functional> or <compare>.

I would like to bring forward a potential change.

Rebase on main + minor changes

Implementing suggestions from @Quuxplusone

Now also deprecates and remove std::result_of

Added _LIBCPP_ENABLE_CXX20_REMOVED_IS_LITERAL_TYPE macro and fixed inconsistencies in some unit tests.

Fix broken is_literal_type.deprecated.fail.cpp unit test

Fixes double commit problem

• Added an extra test for checking deprecation message
• Removed extra lines and other minor problems
• Replaced all instances of

#pragma clang diagnostic ignored "-Wdeprecated-declarations"
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"

with

// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS