[llvm] [ARM] Emit an error when the hard-float ABI is enabled but can't be used (PR #111334)

[Chris Copeland via llvm-commits]

chrisnc wrote:

Now that other broken tests are fixed in #213588, I've updated this PR to handle calling convention overrides and added tests for the new diagnostics. Reviews appreciated!

https://github.com/llvm/llvm-project/pull/111334