[clang-format] Fix crash while reflowing backslash in comments

Summary:
The added test case was currently crashing with an assertion:
```
krasimir@krasimir> cat test.cc                                                                                                                                                              ~
// How to run:
// bbbbb run \
// rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr \
// <log_file> -- --output_directory="<output_directory>"
krasimir@krasimir> ~/work/llvm-build/bin/clang-format test.cc                                                                                                                               ~
clang-format: /usr/local/google/home/krasimir/work/llvm/tools/clang/lib/Format/WhitespaceManager.cpp:117: void clang::format::WhitespaceManager::calculateLineBreakInformation(): Assertion `PreviousOriginalWhitespaceEndOffset <= OriginalWhitespaceStartOffset' failed.
```
The root cause was that BreakableToken was not considering the case of a reflow between an unescaped newline in a line comment.

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D48089

llvm-svn: 334527