[cxx-interop] Fix libc++ tests

These two tests require execution privileges in order to run `check-libcxx-version`, which is used to restrict the tests to a range of libc++ versions. They were failing on `non_executable` CI jobs because of missing `// REQUIRES: executable_test`.

rdar://145821727

Author: egorzhdan

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

@@ -11,6 +11,9 @@
 // This test is specific to libc++ and therefore only runs on Darwin platforms.
 // REQUIRES: OS=macosx || OS=ios
 
+// Since this test runs check-libcxx-version, it requires execution.
+// REQUIRES: executable_test
+
 // CHECK-STD: import CxxStdlib.iosfwd
 // CHECK-STD: import CxxStdlib.string
 

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

@@ -11,6 +11,9 @@
 // REQUIRES: OS=macosx
 // REQUIRES: swift_feature_ImportSymbolicCXXDecls
 
+// Since this test runs check-libcxx-version, it requires execution.
+// REQUIRES: executable_test
+
 // CHECK: enum std {
 // CHECK: enum __1 {