[libc++] Adjust D_LIBCPP_ENABLE_ASSERTIONS defines in exception_guard tests

[ldionne]

Summary

For the assert.FOO.pass.cpp test, we should be passing _LIBCPP_ENABLE_ASSERTIONS=1
for consistency, even though -D_LIBCPP_ENABLE_ASSERTIONS is equivalent.

For the other test, we shouldn't be forcing assertions to be enabled,
since we already have a CI job that enables assertions by default and
will do the right thing.

libcxx/test/libcxx/utilities/assert.exception_guard.no_exceptions.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
 // REQUIRES: has-unix-headers
 // XFAIL: availability-verbose_abort-missing
-// ADDITIONAL_COMPILE_FLAGS: -fno-exceptions -D_LIBCPP_ENABLE_ASSERTIONS
+// ADDITIONAL_COMPILE_FLAGS: -fno-exceptions -D_LIBCPP_ENABLE_ASSERTIONS=1
 
 // ADDITIONAL_COMPILE_FLAGS: -Wno-private-header
 
 #include <__utility/exception_guard.h>
 
 #include "check_assertion.h"
 
 int main(int, char**) {
   TEST_LIBCPP_ASSERT_FAILURE(
       std::__make_exception_guard([] {}), "__exception_guard not completed with exceptions disabled");
 }

libcxx/test/libcxx/utilities/exception_guard.no_exceptions.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
 
-// ADDITIONAL_COMPILE_FLAGS: -fno-exceptions -D_LIBCPP_ENABLE_ASSERTIONS
+// ADDITIONAL_COMPILE_FLAGS: -fno-exceptions
 
 #include <utility>
 
 int main(int, char**) {
   auto guard = std::__make_exception_guard([] {});
   auto guard2 = std::move(guard);
   guard2.__complete();
 
   return 0;
 }