[clang-tidy] Fix naming convention in modernize-use-emplace

Summary: Conform to the llvm naming convention for local variables in modernize-use-emplace check.

Reviewers: Prazek, JonasToth, alexfh

Reviewed By: Prazek, JonasToth, alexfh

Subscribers: cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D32678

llvm-svn: 301780