struct compile_instance needs to be visible to users, since we use
std::unique<compile_instance>. language.c and c-lang.c currently
includes compile-internal.h for this reason, which kind of defeats the
purpose of having an "internal" header file.
Change-Id: Iedffe5f1173b3de7bdc1be533ee2a68e6f6c549f
Reviewed-By: Bruno Larsen <blarsen@redhat.com>