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

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

@kuhar I recently added `DenseSet::insert_range` and wanted to clean up code like:

```
    llvm::DenseSet<file_type> Files;
    Files.insert(PendingOverrides.begin(), PendingOverrides.end());
```

into:

```
    llvm::DenseSet<file_type> Files;
    Files.insert_range(PendingOverrides);
```

But then I thought it would be nice to declare the variable and initialize it at the same time:

```
    llvm::DenseSet<file_type> Files(PendingOverrides);
```

Now, the C++23 way of doing this appears to be:

```
    llvm::DenseSet<file_type> Files(llvm::from_range, PendingOverrides);
```

That's how I've arrived at `llvm::from_range`.  It's not that I'm very keen on having an extra tag in the constructor.

@dwblaikie IIUC, the need for `std::from_range_t` arises because `std::set` in C++23 supports many constructors, including:

```
  std::set<T, Compare> S1(const Compare &Cmp);
  std::set<T, Compare> S2(std::from_range_t, Range &&R);
```

If `std::set` didn't require `std::from_range_t`, and `Compare` had `begin` and `end`, then `S1(Cmp)` would appear to be compatible with both signatures.

Now, our `DenseSet` doesn't support as many constructor signatures as `std::set`, so the ambiguity is not a concern at the moment (unless we want to support some templated code that accepts either `std::set` or `DenseSet` as a template parameter).

By the way, as a data point, our `SmallVector` "deviates" from C++23 in the sense that `SmallVector` allows:

```
SmallVector<int> Vec(Range);
```

whereas the equivalent code with `std::vector` would be:

```
std::vector<int> Vec(std::from_range, Range);
```

Getting back to this PR, I'm totally fine with dropping `llvm::from_range_t` from the constructor proposed in this PR.



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