Fix dependency file escaping.

When writing a dependency (.d) file, if space or # is immediately
preceded by one or more backslashes, escape the backslashes as well as
the space or # character. Otherwise leave backslash alone.
This straddles the fence between BSD Make (which does no escaping at
all, and does not support space or # in filespecs) and GNU Make (which
does support escaping, but will fall back to the filespec as-written
if the escaping doesn't match an existing file).

Differential Revision: http://reviews.llvm.org/D9208

llvm-svn: 237296