Fix enclosing function is not declared 'throws'

Author: weliveindetail

Tests/SwiftFormatPerformanceTests/WhitespaceLinterPerformanceTests.swift

@@ -5,7 +5,7 @@ import XCTest
 @_spi(Testing) import _SwiftFormatTestSupport
 
 final class WhitespaceLinterPerformanceTests: DiagnosingTestCase {
-  func testWhitespaceLinterPerformance() {
+  func testWhitespaceLinterPerformance() throws {
     #if os(Windows)
     // https://github.com/swiftlang/swift-format/issues/939
     throw XCTSkip("This test is flaky on Windows")