[RISCV] Use StringRef::starts_with(char) instead of starts_with(StringRef). NFC

Author: topperc

llvm/lib/TargetParser/RISCVISAInfo.cpp

@@ -177,21 +177,21 @@ bool RISCVISAInfo::addExtension(StringRef ExtName,
 }
 
 static StringRef getExtensionTypeDesc(StringRef Ext) {
-  if (Ext.starts_with("s"))
+  if (Ext.starts_with('s'))
     return "standard supervisor-level extension";
-  if (Ext.starts_with("x"))
+  if (Ext.starts_with('x'))
     return "non-standard user-level extension";
-  if (Ext.starts_with("z"))
+  if (Ext.starts_with('z'))
     return "standard user-level extension";
   return StringRef();
 }
 
 static StringRef getExtensionType(StringRef Ext) {
-  if (Ext.starts_with("s"))
+  if (Ext.starts_with('s'))
     return "s";
-  if (Ext.starts_with("x"))
+  if (Ext.starts_with('x'))
     return "x";
-  if (Ext.starts_with("z"))
+  if (Ext.starts_with('z'))
     return "z";
   return StringRef();
 }