[clang] Add Clang attribute to ensure that fields are initialized explicitly (PR #102040)

[Aaron Ballman via cfe-commits]

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@@ -5214,9 +5215,10 @@ unsigned RecordDecl::getODRHash() {
   // Only calculate hash on first call of getODRHash per record.
   ODRHash Hash;
   Hash.AddRecordDecl(this);
-  // For RecordDecl the ODRHash is stored in the remaining 26
-  // bit of RecordDeclBits, adjust the hash to accomodate.
-  setODRHash(Hash.CalculateHash() >> 6);
+  // For RecordDecl the ODRHash is stored in the remaining
+  // bits of RecordDeclBits, adjust the hash to accommodate.
+  static_assert(sizeof(Hash.CalculateHash()) * CHAR_BIT == 32);
+  setODRHash(Hash.CalculateHash() >> (32 - NumOdrHashBits));
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AaronBallman wrote:

Does anyone have a good intuition for how much this increases the chances of ODR hash collisions? I know we're only stealing one bit, but those bits add up.

https://github.com/llvm/llvm-project/pull/102040