[IR] Drop unnecessary const from a return type (NFC)

Identified with readability-const-return-type.

Author: kazutakahirata

llvm/include/llvm/IR/DebugInfoMetadata.h

@@ -1921,7 +1921,7 @@ class DILocation : public MDNode {
 
   /// Return the linkage name of Subprogram. If the linkage name is empty,
   /// return scope name (the demangled name).
-  const StringRef getSubprogramLinkageName() const {
+  StringRef getSubprogramLinkageName() const {
     DISubprogram *SP = getScope()->getSubprogram();
     if (!SP)
       return "";