[TargetParser] Generate the defs for RISCV CPUs using llvm-tblgen.

This patch removes the file `llvm/include/llvm/TargetParser/RISCVTargetParser.def` and replaces it with a tablegen-generated `.inc` file out of `llvm/lib/Target/RISCV/RISCV.td`.

The module system has been updated to make sure we can build clang/llvm with `-DLLVM_ENABLE_MODULES=On`

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D137517