[Tooling] Add LLVM_NODISCARD to Replacements::merge

[krasimir]

Summary

This patch adds LLVM_NODISCARD to Replacements::merge. I've hit this
several times already.

[ioeric]

Thanks!!!

include/clang/Tooling/Core/Replacement.h

public:
   ///    applying the same replacement twice is equivalent to applying it once.
   /// Replacements with offset UINT_MAX are special - we do not detect conflicts
   /// for such replacements since users may add them intentionally as a special
   /// category of replacements.
   llvm::Error add(const Replacement &R);
 
   /// \brief Merges \p Replaces into the current replacements. \p Replaces
   /// refers to code after applying the current replacements.
-  Replacements merge(const Replacements &Replaces) const;
+  LLVM_NODISCARD Replacements merge(const Replacements &Replaces) const;
 
   // Returns the affected ranges in the changed code.
   std::vector<Range> getAffectedRanges() const;
 
   // Returns the new offset in the code after replacements being applied.
   // Note that if there is an insertion at Offset in the current replacements,
   // \p Offset will be shifted to Offset + Length in inserted text.
   unsigned getShiftedCodePosition(unsigned Position) const;