[llvm] [LangRef] Clarify `align` and `nonnull` interaction with `noundef` in parameter attributes section (PR #224236)

[Ömer Sinan Ağacan via llvm-commits]

osa1 wrote:

@nikic previous sentence:
> The `align` attribute should be combined with the `noundef` attribute to ensure a pointer is aligned, or otherwise the behavior is undefined

Does not say what exactly is undefined when you don't have `noundef`. It could be:
- passing a less aligned value (at the call site)
- using the less aligned value (in the function)

New one:

> If the `align` attribute is combined with the `noundef` attribute, then calling the function with an unaligned value is immediate undefined behavior at the call site, and in the function the value cannot be poison.

Clarifies that `noundef` turns the call site into UB if passed less aligned, and the value becomes poison-free in the function.

https://github.com/llvm/llvm-project/pull/224236