[expr.const] Unclear use of constexpr-referenceable and constexpr-representable due to missing the point

[brevzin]

In [expr.const] we define:

• p3: constexpr-referenceable at a point P
• p4: constexpr-representable at a point P (defined in terms of constexpr-referenceable at P)

However, there are other uses that don't explicitly use a point, the point is simply implied. I think it would add a lot of clarity if we simply explicitly stated a point.

---

The example in p3 is:

struct A {
  int m;
  const int& r;
};
void f() {
  static int sx;
  thread_local int tx;                  // tx is never constexpr-referenceable
  int ax;
  A aa = {1, 2};
  static A sa = {3, 4};
  // The objects sx, ax, and aa.m, sa.m, and the temporaries to which aa.r and sa.r are bound, are constexpr-referenceable.
  auto lambda = [] {
    int ay;
    // The objects sx, sa.m, and ay (but not ax or aa), and the
    // temporary to which sa.r is bound, are constexpr-referenceable.
  };
}

The first comment can say something like "tx is not constexpr-referenceable at any point". The second and third can be changed to something like "The objects xs, ax, [...] are constexpr-referenceable at this point". Alternatively, the example could have comments like // P1 and // P2 and then have text beneath the example saying something like "The objects xs, ax, and aa.m, and the temporary to which sa.r is bound, are constexpr-referenceable at points P1 and P2. The objects ax, aa, and the temporary to which aa.r is bound is constexpr-referenceable at point P1 but not P2." Something in that vein.

---

One of the subbullets in p5 is:

immediately after the initializing declaration of v, the object or reference x declared by v is constexpr-representable, and

Which could be flipped to say

the object or reference x declared by v is constexpr-representable at the point immediately after the initializing declaration of v, and

Just to follow the framing of the definition. Also just reads better anyway I think.