[LangRef] Try to formalize the definition of "odr" in LLVM IR. (#92619)

The current definition is a bit fuzzy... replace it with something
that's somewhat rigorous.

For functions, the definition is pretty narrow; as a consequence of
language-level non-determinism, it's impossible to tell whether two
functions are equivalent, so just embrace the non-determinism. For
constants, we're pretty strict; otherwise you end up concluding
constants can actually change value, which is bad for alias analysis. I
think C++ standard don't allow any non-deterministic operations in
constants, so we should be okay there? Poison is per-byte to allow some
ambiguity in the way padding is defined.