[IR] Drop unnecessary const from return types (NFC)

Identified with const-return-type.

Author: kazutakahirata

llvm/include/llvm/IR/DiagnosticInfo.h

@@ -381,7 +381,7 @@ class DiagnosticInfoWithLocationBase : public DiagnosticInfo {
   /// Return a string with the location information for this diagnostic
   /// in the format "file:line:col". If location information is not available,
   /// it returns "<unknown>:0:0".
-  const std::string getLocationStr() const;
+  std::string getLocationStr() const;
 
   /// Return location information for this diagnostic in three parts:
   /// the relative source file path, line number and column.

llvm/lib/IR/DiagnosticInfo.cpp

@@ -152,7 +152,7 @@ void DiagnosticInfoWithLocationBase::getLocation(StringRef &RelativePath,
   Column = Loc.getColumn();
 }
 
-const std::string DiagnosticInfoWithLocationBase::getLocationStr() const {
+std::string DiagnosticInfoWithLocationBase::getLocationStr() const {
   StringRef Filename("<unknown>");
   unsigned Line = 0;
   unsigned Column = 0;