[libc++] Document warning level at which we're clean

[ldionne]

I don't know how libc++ tests run. Maybe there isn't a test that includes all public headers and builds with -Wall -Wconversion -Wextra -pedantic -Werror? That would be a good test to have.

Does libc++ have any policy which warning levels it's clean at? Probably not really needed since all its headers are supposed to be system headers which disable warnings, so that test could (and probably should?) even use -Weverything with clang.

Originally posted by @nico in #86843 (comment)