Make 'bit', 'logic' and 'time' types unsigned by default

IEEE 1800-2017 6.11.3 says these types are unsigned. Until now these
types were treated as not having a signedness (NOSIGN), and nodes having
these types were later resolved by V3Width to be unsigned. This is a bit
problematic when creating nodes of these types after V3Width. Treating
these types as unsigned from the get go is fine, and actually improves
generated code slightly.

src/V3Ast.h

@@ -504,7 +504,8 @@ public:
     }
     bool isUnsigned() const {
         return m_e == CHANDLE || m_e == EVENTVALUE || m_e == STRING || m_e == SCOPEPTR
-               || m_e == CHARPTR || m_e == UINT32 || m_e == UINT64;
+               || m_e == CHARPTR || m_e == UINT32 || m_e == UINT64 || m_e == BIT || m_e == LOGIC
+               || m_e == TIME;
     }
     bool isFourstate() const {
         return m_e == INTEGER || m_e == LOGIC || m_e == LOGIC_IMPLICIT || m_e == TIME;