Update clang-tidy to enforce naming style on global functions.

**Is your feature request related to a problem? Please describe.**
After some consideration, I've decided our global functions should follow the `lower_case` naming convention as well.

**Describe the solution you'd like**

- [ ] Update the [.clang-tidy](.clang-tidy) file to enfore `lower_case` naming style for global / static functions.
- [ ] Run a clang-debug build (`x64-linux-clang-debug`), (`arm64-osx-clang-debug`), etc... and verify it compiles and `clang-tidy` job passes.

**Describe alternatives you've considered**
I considered keeping the global functions `CamelCase` but I want to adhere to CppCoreGuidelines as much as possible. `CamelCase` will be reserved for types.

**Additional context**