[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

@@ -103,6 +103,11 @@ public struct WarningControlRegionTree {
   /// Root region representing top-level (file) scope
   private var rootRegionNode: WarningControlRegionNode
 
+  /// All of the diagnostic identifiers contained in this tree
+  /// which have at least one occurence with a non-`ignored` behavior
+  /// specifier
+  private var enabledIdentifiers: Set<DiagnosticGroupIdentifier> = []
+
   init(range: Range<AbsolutePosition>) {
     rootRegionNode = WarningControlRegionNode(range: range)
   }
@@ -116,6 +121,9 @@ public struct WarningControlRegionTree {
     let newNode = WarningControlRegionNode(range: range)
     for (diagnosticGroupIdentifier, control) in controls {
       newNode.addWarningGroupControl(for: diagnosticGroupIdentifier, control: control)
+      if control != .ignored {
+        enabledIdentifiers.insert(diagnosticGroupIdentifier)
+      }
     }
     insertIntoSubtree(newNode, parent: rootRegionNode)
   }
@@ -182,6 +190,15 @@ extension WarningControlRegionTree {
   }
 }
 
+extension WarningControlRegionTree {
+  /// Produce an array of diagnostic group identifiers contained in
+  /// this tree which have a behavior specifier other than `ignored`.
+  @_spi(ExperimentalLanguageFeatures)
+  public func getEnabledDiagnosticGroups() -> Set<DiagnosticGroupIdentifier> {
+    return enabledIdentifiers
+  }
+}
+
 /// 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(
       """