[Test] Allow for diagnostic verifier to expect an alternative diagnostic

And fix 'testTraceDependency' where we can have two possibilities for the emitted note.

Author: artemcm

Tests/SwiftDriverTests/ExplicitModuleBuildTests.swift

@@ -2383,11 +2383,8 @@ final class ExplicitModuleBuildTests: XCTestCase {
  try driver.run(jobs: jobs)
 
  diagnostics.expect(.remark("Module 'testTraceDependency' depends on 'A'"))
-#if os(macOS)
- diagnostics.expect(.note("[testTraceDependency] -> [A] -> [A](ObjC)"))
-#else
- diagnostics.expect(.note("[testTraceDependency] -> [C](ObjC) -> [B](ObjC) -> [A](ObjC)"))
-#endif
+ diagnostics.expect(.note("[testTraceDependency] -> [A] -> [A](ObjC)"),
+ alternativeMessage: .note("[testTraceDependency] -> [C](ObjC) -> [B](ObjC) -> [A](ObjC)"))
  }
  }
  }

Tests/SwiftDriverTests/Helpers/AssertDiagnostics.swift

@@ -103,6 +103,7 @@ func assertNoDiagnostics(
 final class DiagnosticVerifier {
  fileprivate struct Expectation {
  let message: Diagnostic.Message
+ let alternativeMessage: Diagnostic.Message?
  let file: StaticString
  let line: UInt
  }
@@ -143,6 +144,10 @@ final class DiagnosticVerifier {
  if diag.matches(expectation.message) {
  self.expected.remove(at: i)
  return
+ } else if let alternativeExpectedMessage = expectation.alternativeMessage,
+ diag.matches(alternativeExpectedMessage) {
+ self.expected.remove(at: i)
+ return
  }
  }
 
@@ -156,6 +161,7 @@ final class DiagnosticVerifier {
  /// is emitted by then, a test assertion will fail.
  func expect(
  _ message: Diagnostic.Message,
+ alternativeMessage: Diagnostic.Message? = nil,
  repetitions: Int = 1,
  file: StaticString = #file, line: UInt = #line
  ) {
@@ -168,7 +174,9 @@ final class DiagnosticVerifier {
  if remaining < 1 { return }
  }
 
- let expectation = Expectation(message: message, file: file, line: line)
+ let expectation = Expectation(message: message,
+ alternativeMessage: alternativeMessage,
+ file: file, line: line)
  self.expected.append(contentsOf: repeatElement(expectation, count: remaining))
  }
  }