[clang] [clang][NFC] Refactor `Selector` to use `PointerIntPair` inside (PR #69916)

[Aaron Ballman via cfe-commits]

================
@@ -809,43 +927,42 @@ class Selector {
   enum IdentifierInfoFlag {
     // Empty selector = 0. Note that these enumeration values must
     // correspond to the enumeration values of DeclarationName::StoredNameKind
-    ZeroArg  = 0x01,
-    OneArg   = 0x02,
+    ZeroArg = 0x01,
+    OneArg = 0x02,
     MultiArg = 0x07,
----------------
AaronBallman wrote:

```suggestion
    OneArg = 0x02,
    // IMPORTANT NOTE: see comments in InfoPtr (below) about this enumerator value.
    MultiArg = 0x07,
```

https://github.com/llvm/llvm-project/pull/69916