[Clang][RISCV] Implement getConstraintRegister for RISC-V

The getConstraintRegister method is used by semantic checking of inline
assembly statements in order to diagnose conflicts between clobber list
and input/output lists. By overriding getConstraintRegister we get those
diagnostics and we match RISC-V GCC's behavior. The implementation is
trivial due to the lack of single-register RISC-V-specific constraints.

Differential Revision: https://reviews.llvm.org/D108624