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

[Min-Yih Hsu via llvm-commits]

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

> range strings will all have a similar beginning and end

oh right, I thought we're only talking about fixed strings.

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