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

[Erich Keane via cfe-commits]

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@@ -5214,9 +5215,9 @@ 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
+  // bit of RecordDeclBits, adjust the hash to accommodate.
+  setODRHash(Hash.CalculateHash() >> (32 - 25));
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erichkeane wrote:

I'd love it if we could put something beyond magic numbers here?  Could we do a sizeof(something?) -  static-constexpr in the `RecordDeclBits` for the num 25?  

Perhaps a `static_assert` as well to make sure the bits is always 32.

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