[clang-tidy] Allow opt-in or out of some commonly occuring patterns in NarrowingConve...

[clang-tidy] Allow opt-in or out of some commonly occuring patterns in NarrowingConversionsCheck.

Within clang-tidy's NarrowingConversionsCheck.
* Allow opt-out of some common occurring patterns, such as:
  - Implicit casts between types of equivalent bit widths.
  - Implicit casts occurring from the return of a ::size() method.
  - Implicit casts on size_type and difference_type.
* Allow opt-in of errors within template instantiations.

This will help projects adopt these guidelines iteratively.
Developed in conjunction with Yitzhak Mandelbaum (ymandel).

Patch by Stephen Concannon!

Differential Revision: https://reviews.llvm.org/D99543