AST: Optimize the layout of AvailabilityRange
#79784
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`AvailabilityRange` is now being used as a currency type in more of the compiler, and some of those uses are in permanent `ASTContext` allocations. The class wraps the `VersionRange` utility, which is itself a wrapper around `llvm::VersionTuple` with some additional storage for representing sentinel values. Even though the two sentinel values can be be represented with just a single bit of additional storage on top of the 16 bytes required to represent `VersionTuple`, because of alignment requirements the sentinel values end up bloating the layout of `VersionRange` by many bytes. To make `AvailabilityRange` and `VersionRange` more efficient to store, we can instead reserve two unlikely `llvm::VersionTuple` bit patterns as the sentinel values instead. The values chosen are the same ones LLVM uses to represent version tuple tombstones and empty keys in a `DenseMap`.
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AvailabilityRangeis now being used as a currency type in more of the compiler, and some of those uses are in permanentASTContextallocations. The class wraps theVersionRangeutility, which is itself a wrapper aroundllvm::VersionTuplewith some additional storage for representing sentinel values. Even though the two sentinel values can be be represented with just a single bit of additional storage on top of the 16 bytes required to representVersionTuple, because of alignment requirements the sentinel values end up bloating the layout ofVersionRangeby many bytes.To make
AvailabilityRangeandVersionRangemore efficient to store, we can instead reserve two unlikelyllvm::VersionTuplebit patterns as the sentinel values instead. The values chosen are the same ones LLVM uses to represent version tuple tombstones and empty keys in aDenseMap.