[llvm] [clang] [RISCV] Deduplicate RISCVISAInfo::toFeatures/toFeatureVector. NFC (PR #76942)

[Saleem Abdulrasool via cfe-commits]

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@@ -470,17 +470,17 @@ std::vector<std::string> RISCVISAInfo::toFeatures(bool AddAllExtensions,
                                                   bool IgnoreUnknown) const {
   std::vector<std::string> Features;
   for (auto const &Ext : Exts) {
-    std::string ExtName = Ext.first;
+    StringRef ExtName = Ext.first;
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compnerd wrote:

Can we just use destructuring yet?

```c++
for (const auto &[ExtName, _] : Exts) {
```

https://github.com/llvm/llvm-project/pull/76942