…oup behavior settings

This library provides support for a mechanism to control the behavior of specific diagnostic groups for a given declaration's lexical scope with the `@warn` attribute.

The syntax tree and its parser do not reason about warning group controls. The syntax tree produced by the parser reprsents the `@warn` attribute in a generic fashion, as it would any other basic attribute on a declaration. The per-declaration nature of the attribute means that for any given declaration's lexical scope, the behavior of a given diagnosic group can be queried by checking for the presence of this attribute in its parent declaration scope.

The `SwiftWarningControl` library provides a utility to determine, for a given source location and diagnostic group identifier, whether or not its behavior is affected by an `@warn` attribute of any of its parent declaration scope.

This utility relies on constructing a warning control tree data structure to represent all warning control lexical scopes in a given source file. This data structure all of the `@warn` diagnostic group behavior controls within the given syntax node, indicating each group's active behavior at a given position.

For example, given code like the following:
```
1:  @warn(Deprecate, as: error)
2:  func foo() {
3:     let a = dep
4:     @warn(Deprecate, as: warning)
5:     func bar() {
6:         let b = dep
7:         @warn(Deprecate, as: ignored)
8:         func baz() {
9:           let c = dep
10:        }
11:        @warn(Deprecate, as: error)
12:        @warn(OtherGroup, as: ignored)
13:        func qux() {
14:            let d = dep
15:            @warn(SomeOtherGroup, as: warning)
16:            func corge() {
17:              let e = dep
18:            }
19:        }
20:     }
21: }
22: func grault() {
23:   let f = dep
24: }
```
the result will be:
```
  - [`Deprecate`:`error`] region within `foo` lexical scope (lines 1-22)
    - [`Deprecate`:`warning`] region within `bar` lexical scope (lines 4-20)
      - [`Deprecate`:`ignored`] region within `baz` lexical scope (lines 7-10)
      - [`Deprecate`:`error`, `OtherGroup`:`ignored`] region within `qux` lexical scope (lines 12-19)
        - [`SomeOtherGroup`:`warning`] region within `corge` lexical scope (lines 15-18)
```

The data structure represents these regions and their nesting relationships as an interval tree where interval nodes can only be nested or disjoint, and where a given interval corresponds to a diagnostic control region for one or more diagnostic group, with a behavior specifier for each.

Intervals cannot partially overlap, and each node's child intervals are always kept sorted. The tree has multiple top-level nodes (roots) representing file-level warning control regions.

Once the tree is computed, lookup of a diagnostic group behavior at a given position is performed by recursively descending into the child node containing the given position (located with a binary search of child nodes of a given parent node). Once the position's depth in the interval tree is reached, we walk back the traversal until we find the first containing region which specifies warning behavior control for the given diagnostic group id.