[clang] Fix #173673 assertion "Output operand lower bound is not zero" with asm output operand with '+Kr' constarint (PR #175470)

[Ulrich Weigand via cfe-commits]

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uweigand wrote:

If we have a CI flag, we probably don't need to set or use `isConstrained` anymore.

https://github.com/llvm/llvm-project/pull/175470