[clang] [CIR] Upstream support for variable length arrays (PR #163297)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

@AmrDeveloper reminded me that when he was working on this before I wanted to try deferring the explicit stack save/restore insertion until lowering. I still think that's a good idea, but after having worked with the test cases for this PR I have some concerns about making it work properly. I'd like to proceed with this PR as is, and I'll try to fix the general cleanup problem I discovered in the incubator before experimenting there with deferring the stack handling.

https://github.com/llvm/llvm-project/pull/163297