[SwiftWarningControl] Keep track of enabled diagnostic group identifiers

And add public API on `WarningControlRegionTree` to query them: `getEnabledDiagnosticGroups`.
This is intended to be used by clients (the compiler) to be able to query whether a given diagnostic group has been enabled in a given source file.

Author: artemcm

Sources/SwiftWarningControl/WarningControlRegions.swift

@@ -103,8 +103,13 @@ public struct WarningControlRegionTree {
   /// Root region representing top-level (file) scope
   private var rootRegionNode: WarningControlRegionNode
 
+  /// All of the diagnostic group identifiers contained in this tree
+  /// which have at least one occurence with a non-`ignored` behavior
+  /// specifier
+  private var enabledGroups: Set<DiagnosticGroupIdentifier> = []
+
   /// Inheritance tree among diagnostic group identifiers
-  let groupInheritanceTree: DiagnosticGroupInheritanceTree
+  private let groupInheritanceTree: DiagnosticGroupInheritanceTree
 
   init(
     range: Range<AbsolutePosition>,
@@ -130,6 +135,10 @@ public struct WarningControlRegionTree {
         let groupIdentifier = groups.removeFirst()
         processedGroups.insert(groupIdentifier)
         newNode.addWarningGroupControl(for: groupIdentifier, control: control)
+        if control != .ignored {
+          enabledGroups.insert(diagnosticGroupIdentifier)
+        }
+
         let newSubGroups = groupInheritanceTree.subgroups(of: groupIdentifier).filter { !processedGroups.contains($0) }
         // Ensure we add a corresponding control to each direct and
         // transitive sub-group of the one specified on this control.
@@ -201,6 +210,15 @@ extension WarningControlRegionTree {
   }
 }
 
+extension WarningControlRegionTree {
+  /// Produce a set of diagnostic group identifiers contained in
+  /// this tree which have a behavior specifier other than `ignored`.
+  @_spi(ExperimentalLanguageFeatures)
+  public func getEnabledDiagnosticGroups() -> Set<DiagnosticGroupIdentifier> {
+    return enabledGroups
+  }
+}
+
 /// A node in the warning control region tree, representing a collection of warning
 /// group controls and references to its nested child regions.
 private class WarningControlRegionNode {

Tests/SwiftWarningControlTest/WarningControlTests.swift

@@ -75,6 +75,40 @@ public class WarningGroupControlTests: XCTestCase {
     )
   }
 
+  func testEnabledGroupIdentifiers() throws {
+    let source =
+      """
+      @warn(Group1, as: warning)
+      @warn(Group2, as: error)
+      @warn(Group3, as: ignored)
+      func foo() {
+        @warn(Group4, as: warning)
+        func bar() {
+          @warn(Group5, as: ignored)
+          @warn(Group6, as: ignored)
+          func baz() {}
+          @warn(Group7, as: error)
+          func qux() {
+            @warn(Group8, as: warning)
+            func corge() {}
+          }
+        }
+      }
+      """
+    var parser = Parser(source)
+    let parseTree = SourceFileSyntax.parse(from: &parser)
+    let warningControlTree = parseTree.warningGroupControlRegionTree()
+    let enabledDiagnosticGroups = warningControlTree.getEnabledDiagnosticGroups()
+    XCTAssertTrue(enabledDiagnosticGroups.contains("Group1"))
+    XCTAssertTrue(enabledDiagnosticGroups.contains("Group2"))
+    XCTAssertFalse(enabledDiagnosticGroups.contains("Group3"))
+    XCTAssertTrue(enabledDiagnosticGroups.contains("Group4"))
+    XCTAssertFalse(enabledDiagnosticGroups.contains("Group5"))
+    XCTAssertFalse(enabledDiagnosticGroups.contains("Group6"))
+    XCTAssertTrue(enabledDiagnosticGroups.contains("Group7"))
+    XCTAssertTrue(enabledDiagnosticGroups.contains("Group8"))
+  }
+
   func testNominalDeclWarningGroupControl() throws {
     try assertWarningGroupControl(
       """