[clang-tidy] modernize-use-emplace: remove unnecessary make_pair calls

Summary:
When there is a push_back with a call to make_pair, turn it into emplace_back and remove the unnecessary make_pair call.

Eg.

```
std::vector<std::pair<int, int>> v;
v.push_back(std::make_pair(1, 2)); // --> v.emplace_back(1, 2);
```

make_pair doesn't get removed when explicit template parameters are provided, because of potential problems with type conversions.

Reviewers: Prazek, aaron.ballman, hokein, alexfh

Reviewed By: Prazek, alexfh

Subscribers: JDevlieghere, JonasToth, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D32395

llvm-svn: 301651