[clang] [clang] Reduce the size of ParsedAttributesView and AttributePool (PR #148726)

[Erich Keane via cfe-commits]

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@@ -808,7 +808,7 @@ class AttributePool {
 
 class ParsedAttributesView {
   friend class AttributePool;
-  using VecTy = llvm::SmallVector<ParsedAttr *>;
+  using VecTy = llvm::SmallVector<ParsedAttr *, 2>;
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erichkeane wrote:

Curious why '2' was found?  Did you see perf-benefit from 2 over 0(aka, `std::vector`)?  Presumably the benefit of cache locality isn't really applicable as attributes are comparatively rare, so 'while we're at it', perhaps saving twice as much would be valuable? 

https://github.com/llvm/llvm-project/pull/148726