devel/llvm-{cheri,morello}: align warning with base

FreeBSD's base llvm currently makes -Wimplicit-int and
-Wimplicit-function-declaration not produce errors by default (see
c3450ad127e94d3fa67ae337310e59c002f51bfc) which means a chunk
of the ports collection depends on this behavior. Until this is resolve
we need to mirror that behavior on CheriBSD.

Sponsored by: DARPA, AFRL