[clang] [clang] Fix AArch64 -ffixed-x help text (PR #188911)

[Carlo Cabrera via cfe-commits]

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carlocab wrote:

Can you add analogous tests for the other unsupported registers? This test file already has full coverage for the supported registers, we should complete that by filling in tests for all unsupported registers.

https://github.com/llvm/llvm-project/pull/188911