c++: More cleanups for modern C++

c++: More cleanups for modern C++

Here are some more places where we can declare variables at the
assignment point, rather than use C89.  Also, let's name our variables
by what they contain -- the register allocator is perfectly able to
track liveness for us.

	gcc/cp/
	* decl.c (decls_match): Move variables into scopes
	they're needed in.
	(duplicate_decls): Use STRIP_TEMPLATE.
	(build_typename_type): Move var decls to their assignments.
	(begin_function_body): Likewise.
	* decl2.c (get_guard): Likewise.
	(mark_used): Use true for truthiness.
	* error.c (dump_aggr_type): Hold the decl in a var called
	'decl', not 'name'.