[interop][test] only run libc++ module interface printing test when the SDK has libc++

Author: hyp

test/Interop/Cxx/stdlib/print-libcxx-module-interface.swift

@@ -1,6 +1,7 @@
 // RUN: %target-swift-ide-test -print-module -module-to-print=std -I %S/Inputs -source-filename=x -enable-experimental-cxx-interop -enable-objc-interop -module-print-submodules | %FileCheck %s
 
 // REQUIRES: OS=macosx
+// REQUIRES: libcxx-in-sdk
 
 // CHECK: enum std {
 // CHECK-NEXT: enum __1 {

test/Interop/lit.local.cfg

@@ -12,6 +12,10 @@ clang_opt = clang_compile_opt
 is_cf_options_interop_updated = True
 
 if config.variant_sdk and config.variant_sdk != "":
+    # Check if libc++ is present in the SDK or not.
+    if config.target_sdk_libcxx_path and os.path.exists(config.target_sdk_libcxx_path):
+        config.available_features.add('libcxx-in-sdk')
+
     # Check if CF_OPTIONS macro has been updated to be imported into Swift in C++ mode correctly.
     cf_avail_path = os.path.join(config.variant_sdk, 'System', 'Library', 'Frameworks', 'CoreFoundation.framework', 'Versions', 'A', 'Headers', 'CFAvailability.h')
     if os.path.exists(cf_avail_path):