Don't remove backslashes from arguments unless the following char is recognized.

This fixes file paths on Windows, as you can now write, for example,
file d:\foo\bar.txt, but does not break the case that this tokenization
logic was originally designed for, which is to allow escaping of things
like quotes and double quotes, so that all of the escapable characters
can appear in the same string together.

Reviewed by: Jim Ingham, Greg Clayton
Differential Revision: http://reviews.llvm.org/D7018

llvm-svn: 226587