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

[Ilya Biryukov via cfe-commits]

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@@ -1682,7 +1690,7 @@ class DeclContext {
 
     /// True if a valid hash is stored in ODRHash. This should shave off some
     /// extra storage and prevent CXXRecordDecl to store unused bits.
-    uint64_t ODRHash : 26;
+    uint64_t ODRHash : 25;
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ilya-biryukov wrote:

Oh, wow, so we have to chip off the bits from an ODR hash when we add a new bit to class state?
This is unfortunate, especially given that it is somewhat hard to measure which trade-offs this entails? (How many bits of ODR hash is too little?)

I don't think it is a reason to block this PR, but it would be nice to come up with a way to measure which number of bits we cannot go under.

A major comment: you also have to update the place that sets the ODRHash. For some reason, we prefer to have the top 26 bits, not the low 26 bits, see: https://github.com/llvm/llvm-project/blob/fb6feb86a7dc324dcb2516397ba3fc5fe05ea584/clang/lib/AST/Decl.cpp#L5219

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