[llvm] [RISCV][NFC] Adopt DiagnosticString interface (PR #126290)

[Sam Elliott via llvm-commits]

================
@@ -150,12 +151,26 @@ def SPMem : MemOperand<SP>;
 
 def GPRCMem : MemOperand<GPRC>;
 
----------------
lenary wrote:

> I wonder how much work it be to introduce "DiagnosticCode" to allow us to programmatically generate the string like we do now

This feels like overenginnering. I'm like down three side-rabbit-holes now, and going to retreat a bit.

I will undo the tablegen crimes, the C++ is better for craig's reasons, but where we have fixed strings this should make things easier.

https://github.com/llvm/llvm-project/pull/126290