[llvm] [ItaniumDemangle] reject A-F in FP literals (PR #83061)

[Ryan Prichard via llvm-commits]

rprichard wrote:

It looks like the `Flang :: Fir/memory-allocation-opt.fir` Windows test is failing on other GitHub PRs, too, so it's unrelated to this change.

https://github.com/llvm/llvm-project/pull/83061