Add cplusplus_driver lit feature

This will be used to mark tests as REQUIRES: cplusplus_driver if
they cannot be easily made to work with swift-driver due to architectural differences
(for example, the lack of -driver-print-actions), and the test cases have been ported to XCTest

This feature is enabled if the swift_driver param is NOT passed to lit.py

Author: owenv

test/lit.cfg

@@ -617,6 +617,9 @@ else:
 if 'swift_evolve' in lit_config.params:
     config.available_features.add("swift_evolve")
 
+if not 'swift_driver' in lit_config.params:
+    config.available_features.add("cplusplus_driver")
+
 # Enable benchmark testing when the binary is found (has fully qualified path).
 if config.benchmark_o != 'Benchmark_O':
     config.available_features.add('benchmark')