[clang-format] Don't reflow across comment pragmas.

Summary:
The comment reflower wasn't taking comment pragmas as reflow stoppers. This patch fixes that.

source:
```
// long long long long
// IWYU pragma:
```
format with column limit  = 20 before:
```
// long long long
// long IWYU pragma:
```
format with column limit  = 20 after:
```
// long long long
// long
// IWYU pragma:
```

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D29450

llvm-svn: 293898