Proposed LangRef change to relax the const'ness of vscale

I want to raise awareness of ⚙ D134648 [LangRef] Update text for vscale to be more flexible but maintain original intent.. The reason for the change comes from ⚙ D131562 [AArch64][SME] Document SME ABI implementation in LLVM where there’s a real world use case for changing vscale at runtime.

My intent is to minimally relax the requirements in such a way that existing use cases remain valid, whilst providing a route to handle a runtime change of vscale. I’m hoping to move the extent of const’ness to function boundaries rather than the whole runtime of a binary.

One issue relating to ConstantExprs has already been identified. This seems worth fixing despite this proposed langref change and is being investigated.