[Tooling] Add LLVM_NODISCARD to Replacements::merge

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

Reviewers: ioeric

Reviewed By: ioeric

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D36149

llvm-svn: 309689