[interop] check if CI can find libc++ from swift-ide-test finally

Author: hyp

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

@@ -1,4 +1,4 @@
-// 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
+// RUN: %target-swift-ide-test -print-module -module-to-print=std -I %libcxx-in-sdk-path -source-filename=x -enable-experimental-cxx-interop -enable-objc-interop -module-print-submodules | %FileCheck %s
 
 // REQUIRES: OS=macosx
 // REQUIRES: libcxx-in-sdk

test/Interop/lit.local.cfg

@@ -15,6 +15,7 @@ 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')
+        config.substitutions.insert(0, ('%libcxx-in-sdk-path', config.target_sdk_libcxx_path))
 
     # 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')