fix julia_asserts to depend only on julia build mode

[JeffBezanson]

Assertions in parts of our C++ code seemed to always be enabled; for example in #41157 where the assertion happens in all builds. This moves some includes and fixes the logic in julia_assert.h to make assertions in our code depend only on JL_NDEBUG.
A couple remaining questions about assertions:

• Should we inspect llvm-config --assertion-mode and pass -DNDEBUG (in addition to the existing -DJL_NDEBUG) if LLVM assertions are off?
• We could look into using EnableABIBreakingChecksin LLVM, which adds more checks at the cost of requiring the flag to match in callers.