[clang] [clang-tools-extra] [clang] Concepts: support pack expansions for type constraints (PR #132626)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

This is a compile-time improvement overall.

The reverted patch adds a 0.07 cost to stage2-clang, which we avoid with the new approach:
Commit	stage1-O3	stage1-ReleaseThinLTO	stage1-ReleaseLTO-g	stage1-O0-g	stage2-O3	stage2-O0-g	stage2-clang
[9a089c5fbb](https://github.com/llvm/llvm-project/commit/9a089c5fbb4f84da58bca7559f15149b4d3b83d6)	60976M (+0.00%)	77222M (-0.00%)	89531M (-0.01%)	18999M (+0.01%)	53187M (+0.01%)	16612M (-0.02%)	34778529M (+0.01%)
[C](https://llvm-compile-time-tracker.com/compare.php?from=17bbac4d1f447a448168de629f44d8c227d468e5&to=e7ecd60e5fb7a9a14f7d85ab23587faadb543819&stat=instructions:u)		[e7ecd60e5f](https://github.com/llvm/llvm-project/commit/e7ecd60e5fb7a9a14f7d85ab23587faadb543819)	60974M (-0.00%)	77223M (+0.01%)	89537M (+0.01%)	18998M (-0.01%)	53181M (-0.00%)	16615M (+0.01%)	34773327M (-0.07%)
[C](https://llvm-compile-time-tracker.com/compare.php?from=fb44c54d656135a306e403083b5c1eaae9078757&to=17bbac4d1f447a448168de629f44d8c227d468e5&stat=instructions:u)		[17bbac4d1f](https://github.com/llvm/llvm-project/commit/17bbac4d1f447a448168de629f44d8c227d468e5)	60975M (-0.01%)	77211M (-0.01%)	89525M (-0.01%)	19000M (-0.01%)	53184M (-0.03%)	16614M (-0.04%)	34798333M (-0.01%)

https://github.com/llvm/llvm-project/pull/132626