[libc++] Granularize <exception> includes

[ldionne]

I am fine with this in principle, but there seems to be a bit of work needed still. LGTM once CI is green. Also, when you remove transitive includes from a public header, let's add it to a _LIBCPP_REMOVE_TRANSITIVE_INCLUDES block (even if it might not be needed strictly speaking because the transitive include still exists via a private header). That documents what the public header is including and for what reasons (e.g. _LIBCPP_REMOVE_TRANSITIVE_INCLUDES implies the reason is legacy), and that's useful to know.

libcxx/include/__functional/function.h

34–35

Are those not needed at all?

libcxx/include/functional

546

Don't we need to add this below to the _LIBCPP_REMOVE_TRANSITIVE_INCLUDES block?

libcxx/include/variant

242

_LIBCPP_REMOVE_TRANSITIVE_INCLUDES?

[philnik]

address comments

[philnik]

Try to fix CI