[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

Author: eopXD

clang/include/clang/Support/RISCVVIntrinsicUtils.h

@@ -92,15 +92,22 @@ enum class TypeModifier : uint8_t {
   LLVM_MARK_AS_BITMASK_ENUM(LMUL1),
 };
 
-struct Policy {
-  bool IsUnspecified = false;
+class Policy {
+public:
   enum PolicyType {
     Undisturbed,
     Agnostic,
   };
-  PolicyType TailPolicy = Agnostic;
-  PolicyType MaskPolicy = Agnostic;
+  bool IsUnspecified = false;
+
+private:
+  // The default assumption for an RVV instruction is TAMA, as an undisturbed
+  // policy generally will affect the performance of an out-of-order core.
+  const PolicyType TailPolicy = Agnostic;
+  const PolicyType MaskPolicy = Agnostic;
   bool HasTailPolicy, HasMaskPolicy;
+
+public:
   Policy(bool HasTailPolicy, bool HasMaskPolicy)
       : IsUnspecified(true), HasTailPolicy(HasTailPolicy),
         HasMaskPolicy(HasMaskPolicy) {}