Add __traits getBitfieldOffset and getBitfieldWidth

This completes the introspection capabilities of builtin bitfields. For example:

---
struct S
{
    int a,b;
    int :2, c:3;
}

static assert(__traits(getBitfieldOffset, S.b) == 0);
static assert(__traits(getBitfieldOffset, S.c) == 2);
static assert(__traits(getBitfieldWidth, S.b) == 32);
static assert(__traits(getBitfieldWidth, S.c) == 3);
---