[llvm] [ADT] Add DenseSet(llvm::from_t, Range) (PR #131832)

[Jakub Kuderski via llvm-commits]

kuhar wrote:

Hi @kazutakahirata,

I thought about this over the week and I think this proposal is the most reasonable one after all. The issue with adding a range constructor is that it may lead to unintentional copies like with `DesneSet<int64_t> foo = bar();` where `bar()` returns `DenseSet<int>` or some other compatible type.

With CTAD, `from_range` seems more reliable for both definition styles: `DenseSet<int64_t> foo = bar();` (no implicit copy) and `auto foo = DenseSet(from_range, bar())` (for generic code). We can also always introduce a helper function `to_dense_set` if typing `from_range` becomes too verbose.

https://github.com/llvm/llvm-project/pull/131832