c++: Fix ENABLE_SCOPE_CHECKING printing

The lists of scope kinds used by ENABLE_SCOPE_CHECKING don't seem to
have been updated in a long while, causing ICEs and confusing output.
This patch brings the list into line.

Additionally, the comment on 'explicit_spec_p' says that the flag is
only valid if kind is 'sk_template_parms', so we rewrite the condition
to be more obviously correct here.

gcc/cp/ChangeLog:

	* name-lookup.h (enum scope_kind): Add 'sk_count'.
	* name-lookup.cc (cp_binding_level_descriptor): Add missing
	scope kinds. Add assertion that the list is up to date. Fix
	handling of explicit_spec_p.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>