Add option to align escaped newlines left.

This enables formattings like:

  #define A   \
    int aaaa; \
    int b;    \
    int ccc;  \
    int dddddddddd;

Enabling this for Google/Chromium styles only as I don't know whether it
is desired for Clang/LLVM.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180253 91177308-0d34-0410-b5e6-96231b3b80d8