[RISCV] Use StringRef in a range-based for loop (NFC) (#144243)

When we iterate over std::vector<std::string>, we can directly assign
each element to StringRef.  We do not need to go through a separate
statement.

Author: kazutakahirata

llvm/lib/TargetParser/RISCVISAInfo.cpp

@@ -449,8 +449,7 @@ RISCVISAInfo::parseFeatures(unsigned XLen,
   assert(XLen == 32 || XLen == 64);
   std::unique_ptr<RISCVISAInfo> ISAInfo(new RISCVISAInfo(XLen));
 
-  for (auto &Feature : Features) {
-    StringRef ExtName = Feature;
+  for (StringRef ExtName : Features) {
     assert(ExtName.size() > 1 && (ExtName[0] == '+' || ExtName[0] == '-'));
     bool Add = ExtName[0] == '+';
     ExtName = ExtName.drop_front(1); // Drop '+' or '-'