[llvm] [AArch64][Win] Account for the fixed object area when classifying CSR… (PR #222111)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

The fix looks reasonable to me. The PR description still looks like a verbose AI message. Can you rewrite it into a typical LLVM commit message? A few lines normally is enough, e.g.:

```
[AArch64] Account for fixed objects when resolving frame offsets

Include the fixed object area in the callee-saved threshold used by 
resolveFrameOffsetReference. This prevents callee-saved slots from 
being incorrectly addressed through the base pointer (i.e., like a 
local) in stack-realigned Windows frames.
```

https://github.com/llvm/llvm-project/pull/222111