[3/3][RISCV][POC] Model vxrm in C intrinsics for RVV fixed-point instruction vaadd, vasub

Depends on D151396.

This is the 3rd patch of the patch-set. For the cover letter of the
patch-set, please checkout D151395.

This commit consists of change in both clang front-end and RISC-
back-end.

In the front-end, this commit adds an additional operand to the C
intrinsics of `vaadd`, `vaaddu`, `vasub`, and `vasubu`, that models
the control of the rounding mode.

In the back-end, using `vaadd` as an example, this commit replaces the
existing `int.riscv.vaadd.*` with `int.riscv.vaadd.rm.*` that was
introduced in the previous patch, with the extra operand that models
the control of the rounding mode (`vxrm`) for RVV fixed-point
intrinsics.

Note: The first 3 commit of the patch-set shows the intent to model the
rounding mode for fixed-point intrinsics by applying change to
`vaadd`, `vaaddu`, `vasub`, and `vasubu`. The proceeding patch will
apply the change to the rest of the other fixed-point instructions.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D151397