[RISCV] Merge RISCV::parseCPUKind and RISCV::checkCPUKind.

Similar for RISCV::parseTuneCPU and RISCV::checkTuneCPUKind.

This makes the CPUKind enum no longer part of the API. It wasn't
providing much value. It was only used to pass between the two
functions.

By removing it, we can remove a dependency on a tablegen generated
file from the RISCVTargetParser.h file. Then we can remove a
dependency from several CMakeLists.txt.