[clang] [CIR] Handle FunctionToPointerDecay casts (#153657) (PR #154060)

[Justin Riddell via cfe-commits]

Arghnews wrote:

@andykaylor thank you for the suggestion, have rebased and rerun with latest and it works fine now

Have updated the tests - I wasn't sure how closely the test should watch the cir output (whether too close a match is an issue, or not), so please check. I force pushed anyway so that if this is fine, it can be committed as is

https://github.com/llvm/llvm-project/pull/154060