[clang] Turn 'counted_by' into a type attribute and parse it into 'CountAttributedType' (PR #78000)

[Aaron Ballman via cfe-commits]

================
@@ -2000,6 +2001,21 @@ class alignas(TypeAlignment) Type : public ExtQualsTypeCommonBase {
     unsigned NumExpansions;
   };
 
+  class CountAttributedTypeBitfields {
+    friend class CountAttributedType;
+
+    LLVM_PREFERRED_TYPE(TypeBitfields)
+    unsigned : NumTypeBits;
+
+    /// The limit is 15.
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AaronBallman wrote:

I think `NumTypesBits` is 19 (it's not based on the underlying type of the enum, `llvm::BitWidth` is clever there and uses the maximal enum value to calculate the number of bits needed), this structure adds 6 more bits, so that's up to 25 bits. I would expect that means we have 7 bits leftover so we keep this to a 32-bit value.

https://github.com/llvm/llvm-project/pull/78000