[clang-format] Don't break lines after pragma region

We have autogenerated pragma regions in our code
which where awkwardly broken up like this:

```
#pragma region foo(bar : hello)
```
becomes

```
#pragma region foo(bar \
                   : hello)
```

This fixes the problem by adding region as a keyword
and handling it the same way as pragma mark

Reviewed By: curdeius

Differential Revision: https://reviews.llvm.org/D125961