[libc++] Remove unnecessary main() function in .compile.pass.cpp and .verify.cpp...

[libc++] Remove unnecessary main() function in .compile.pass.cpp and .verify.cpp tests

We pretty consistently don't define those cause they are not needed,
and it removes the potential pitfall to think that these tests are
being run. This doesn't touch .compile.fail.cpp tests since those
should be replaced by .verify.cpp tests anyway, and there would be
a lot to fix up.

As a fly-by, I also fixed a bit of formatting, removed a few unused
includes and made some very minor, clearly NFC refactorings such as
in allocator.traits/allocator.traits.members/allocate.verify.cpp where
the old test basically made no sense the way it was written.

Differential Revision: https://reviews.llvm.org/D146236