Make <assert.h> consistently available in frontend and backend

Make <assert.h> consistently available in frontend and backend

Previously, c.h made <assert.h> only available in frontends (#ifdef
FRONTEND), which was probably reasonable, because the only thing it
would give you is assert(), which you generally shouldn't use in the
backend.  But with C11, <assert.h> also makes available
static_assert(), which would be useful everywhere.  So this patch
moves <assert.h> to the commonly available header files in c.h and
fixes a small complication in regcustom.h that resulted from that.

Co-authored-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CA%2BhUKGKvr0x_oGmQTUkx%3DODgSksT2EtgCA6LmGx_jQFG%3DsDUpg%40mail.gmail.com