libswift: if libswift is enabled, add ‘libswift’ to the lit features

This lets tests run conditionally on libswift:
// REQUIRES: libswift

Author: eeckstein

test/CMakeLists.txt

@@ -153,6 +153,10 @@ if(NOT CMAKE_CFG_INTDIR STREQUAL ".")
        "--param" "build_mode=${CMAKE_CFG_INTDIR}")
 endif()
 
+if(SWIFT_TOOLS_ENABLE_LIBSWIFT)
+  list(APPEND SWIFT_LIT_ARGS "--param" "libswift")
+endif()
+
 if (LLVM_USE_SANITIZER STREQUAL "Address")
   set(SWIFT_ASAN_BUILD TRUE)
 endif()

test/lit.cfg

@@ -254,6 +254,9 @@ native_swift_tools_path = lit_config.params.get('native_swift_tools_path')
 if native_swift_tools_path is not None:
     append_to_env_path(native_swift_tools_path)
 
+if lit_config.params.get('libswift', None) is not None:
+    config.available_features.add('libswift')
+
 ###
 
 # Discover the Swift binaries to use.