Add NDK via --sysroot in check-cxx-header-in-clang invocations to fix Interop/SwiftToCxx tests

Author: weliveindetail

test/lit.cfg

@@ -2756,6 +2756,7 @@ config.substitutions.insert(0, ('%check-c-header-in-clang',
 # Verifies that a C++ file can be compiled without warnings except for some exceptions.
 config.substitutions.insert(0, ('%check-cxx-header-in-clang',
   '%clangxx -fsyntax-only -x c++-header ' + extraCheckWarningFlags +
+  ' --sysroot %r ' % config.variant_sdk +
   # Use standard header/framework search paths.
   '-F %%clang-importer-sdk-path/frameworks '
   '-I %%clang-include-dir %%cxx-stdlib-include '