Fix declaration_specifiers() handling of typedef name shadowed by NS_SYMBOL

Fix declaration_specifiers() handling of typedef name shadowed by NS_SYMBOL

Doing lookup_symbol() with NS_TYPEDEF will happily skip the redeclarations
of the same identifier with NS_SYMBOL.  We need to check that we are not
dealing with something like
typedef int T;
void f(int T)
{
	static T a; /* not a valid declaration - T is not a typedef name */
or similar (e.g. enum member shadowing a typedef, etc.).

While we are at it, microoptimize similar code in lookup_type() - instead
of sym->namespace == NS_TYPEDEF we can do sym->namespace & NS_TYPEDEF;
the former will turn into "fetch 32bit value, mask all but 9 bits, compare
with NS_TYPEDEF", the latter - "check that one bit in 32bit value is set".
We never mix NS_TYPEDEF with anything in whatever->namespace, so the
tests are equivalent.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Christopher Li <sparse@chrisli.org>