[libc++] Update and normalize the "all the headers" tests.

Some C++20 headers weren't added properly to all three of these
test files. Add them, and take the time to normalize the formatting
so that

    diff <(grep '#include' foo.cpp) <(grep '#include' bar.cpp)

shows no diffs (except that `no_assert_include` deliberately
excludes `<cassert>`).

- Add macro guards to <{barrier,latch,semaphore}>.
- Add macro guards to <experimental/simd>.
- Remove an include of <cassert> from <semaphore>.
- Instead, include <cassert> in the semaphore tests.

Differential Revision: https://reviews.llvm.org/D92525