[clang-format] avoid introducing multiline comments

In C++ with -Werror=comment, multiline comments are not allowed.
clang-format could accidentally introduce multiline comments when reflowing.
This adapts clang-format to not introduce multiline comments by not allowing a
break after `\`. Note that this does not apply to comment lines that already are
multiline comments, such as comments in macros.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D90949