[Test] Define availability macro SwiftStdlib 5.5 for all tests

Author: xymus

test/lit.cfg

@@ -399,6 +399,12 @@ swift_version = lit_config.params.get('swift-version',
 lit_config.note('Compiling with -swift-version ' + swift_version)
 config.swift_test_options = '-swift-version ' + swift_version
 
+# Define a macro for the next release OS version.
+# This macro can be used in code as `@available(SwiftStdlib 5.5, *)`.
+swift_stdlib_macro = '\'SwiftStdlib 5.5:macOS 9999, iOS 9999, watchOS 9999, tvOS 9999\''
+config.swift_frontend_test_options += ' -define-availability ' + swift_stdlib_macro
+config.swift_driver_test_options += ' -Xfrontend -define-availability -Xfrontend ' + swift_stdlib_macro
+
 differentiable_programming = lit_config.params.get('differentiable_programming', None)
 if differentiable_programming is not None:
     config.available_features.add('differentiable_programming')