[clang] [Clang][Sema][NFCI] Simplify `resolveAllocationOverload()` (PR #203824)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> > I would _really_ like to get rid of the operations over stack allocated AST nodes.
> 
> This deserves its own atomic patch, separate from other changes, no?

This PR is intended to simplify resolveAllocationOverload(), it is a restructuring, and removing the stack allocated nodes is part of that simplification -- if we want to simplify this code, we need to simplify it.

The complexity of this code is a consequence of _structural_ deficiencies, the current PR does not address those structural issues, it's trying to "simplify" the code around those deficiencies. The result is that I do not believe that it is meaningfully changing the complexity of the code, it is essentially shuffling the complexity around.

https://github.com/llvm/llvm-project/pull/203824