[clang] [clang][Sema] Handle undeduced auto types in HeuristicResolver (PR #124236)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> Thanks, though having to nest five levels of 'if's seems a bit awkward

I'll follow this up with a patch to implement the `FIXME`, it will almost certainly involve factoring out a new helper function so the nesting should be reduced.

https://github.com/llvm/llvm-project/pull/124236