[15/15][Clang][RISCV][NFC] Set data member under Policy as constants

The object is now correct by construction.

This is the 15th commit of a patch-set that aims to change the default policy
for RVV intrinsics from TAMU to TAMA.

Please refer to the cover letter in the 1st commit (D141573) for an
overview.

Depends on D141793.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D141796