[2/15][Clang][RISCV][NFC] Rename Policy::IsPolicyNone to IsUnspecifed

The original naming is inaccurate. An RVV intrinsic always comes with
a corresponding policy behavior. When the policy is unspecified for an
intrinsic, its policy behavior should be it's default assumption.

This is the 2nd 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.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D141574