This debug instruction partially updates a local variable with SSA values.

This is typically used to update a `DebugLocalVariable` of a composite type, which holds structure member offsets from a base address.
To handle these, we allocate shadow memory to hold a copy of the entire variable in contiguous memory and have the `DebugLocalVariable` point to this memory. Whenever we encounter a `DebugValue`, we copy the necessary fields to the
shadow memory.

This approach is simplier and more performant than attempting to fiddle with `vk::dbg::Value`s for each `DebugValue`.

Bug: b/148401179
Change-Id: I6aafce1f7553f4a3a97bbf84e6e9dac97ce71a68
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/48429Reviewed-by: Jaebaek Seo <jaebaek@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>