[clang] [llvm] [RISCV] Improve error message when the extension is not supported (PR #83989)

[David Blaikie via cfe-commits]

dwblaikie wrote:

perhaps the llvm libSupport prats of this change should be unit tested in LLVM, rather than only tested indirectly in clang?

https://github.com/llvm/llvm-project/pull/83989