[clang] [CIR] Upstream support for Variable length Array (PR #137233)

[Amr Hesham via cfe-commits]

AmrDeveloper wrote:

This is a WIP patch for VLA support.

When I moved the emitting StackOps to be before lowering and created `OpRewritePattern` for `AllocaOp`, that means that Dead code elimination will be performed for each AllocaOp and in our test files, We can't test unused variables and Struct, Unions, TypeDef test must be modified to add usage for each variable

If this approach is accepted, I will continue updating test files and cleaning the PR, but that means also that any init PR for supporting a new type must at least support `sizeof` for that type so it can be tested

@andykaylor @bcardosolopes What do you think? 🤔 

https://github.com/llvm/llvm-project/pull/137233