[clang-format] Convert AlignEscapedNewlinesLeft to an enum, adding

DontAlign

This converts the clang-format option AlignEscapedNewlinesLeft from a
boolean to an enum, named AlignEscapedNewlines, with options Left (prev.
true), Right (prev. false), and a new option DontAlign.

When set to DontAlign, the backslashes are placed just after the last token in each line:
  #define EXAMPLE \
    do { \
        int x = aaaaa; \
        int b; \
        int dddddddddd; \
    } while (0)

Patch by jtbandes. Thank you!

llvm-svn: 302428