[Support] Fix argv string escape bug on Windows

Summary:
This is http://llvm.org/PR15802.  Backslashes preceding double quotes in
arguments must be escaped.  The interesting bit is that all other
backslashes should *not* be escaped, because the un-escaping logic is
only triggered by the presence of a double quote character.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D705