Change 'v1' float and int code to fall back to v0

Change 'v1' float and int code to fall back to v0

While working on another patch, I discovered that the libcc1 plugin
code never did version negotiation correctly.  So, the patches to
introduce v1 never did anything -- the new code, as far as I know, has
never been run.

Making version negotiation work shows that the existing code causes
crashes.  For example, safe_lookup_builtin_type might return
error_mark_node in some cases, which the callers aren't prepared to
accept.

Looking into it some more, I couldn't find any justification for this
v1 code for the C compiler plugin.  Since it's not run at all, it's
also clear that removing it doesn't cause any regressions in gdb.

However, rather than remove it, this patch changes it to handle
ERROR_MARK better, and then to fall back to the v0 code if the new
code fails to find the type it's looking for.

libcc1

	* libcc1plugin.cc (safe_lookup_builtin_type): Handle ERROR_MARK.
	(plugin_int_type): Fall back to plugin_int_type_v0.
	(plugin_float_type): Fall back to plugin_float_type_v0.