[libc++] Adjust D_LIBCPP_ENABLE_ASSERTIONS defines in exception_guard tests

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.

Differential Revision: https://reviews.llvm.org/D148736