Summary:
This PR adds the final set of clang-tidy checks we should add for our codebase: a last set of performance-related checks. Most fixes here are around changing `auto` to `const auto&` in a few places where unnecessary copies were made, and adding `reserve()` calls before loops doing repeated `push_back()`. Also a few cases of calling `std::string::find` with a single-character string literal instead of a single char, which uses a less efficient string search algorithm meant for searching larger substrings.

![image](https://user-images.githubusercontent.com/6429851/49978940-adc1a780-ff01-11e8-99da-a4e431361f07.png)

ezyang apaszke
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15198

Differential Revision: D13468797

Pulled By: goldsborough

fbshipit-source-id: 2bed1ea1c7c162b7f3e0e1026f17125e88c4d5b2