[llvm] [DenseMap] Share rehash and grow for relocatable bucket types. NFC (PR #225018)

[Fangrui Song via llvm-commits]

================
@@ -56,6 +56,9 @@ template <typename T, typename Enable = void> struct DenseMapInfo {
 // Provide DenseMapInfo for all pointers. Avoid requiring T to be complete so
 // clients can instantiate DenseMap<T*, ...> with forward declared key types.
 template <typename T> struct DenseMapInfo<T *> {
+  // Select InlinePtrHash in DenseMap.cpp.
+  using PointerValueHash = DenseMapInfo;
----------------
MaskRay wrote:

`using PointerValueHash = DenseMapInfo;` (type alias) is intentional.

```cpp
// X * cannot take the `hashesPointerValue` code path.
template <> struct DenseMapInfo<X *> : DenseMapInfo<Y *> {
  static unsigned getHashValue(const X *P) { return P->V * 37; }
  static bool isEqual(const X *L, const X *R) { return L == R; }
};
```

== alias   hashesPointerValue<X *, DenseMapInfo<X *>> = 0   found 200 of 200 after grow
== bool    hashesPointerValue<X *, DenseMapInfo<X *>> = 1   found 0 of 200 after grow
```

https://github.com/llvm/llvm-project/pull/225018