[clang-format] Align preprocessor comments with #

Summary:
r312125, which introduced preprocessor indentation, shipped with a known
issue where "indentation of comments immediately before indented
preprocessor lines is toggled on each run". For example these two forms
toggle:

  #ifndef HEADER_H
  #define HEADER_H
  #if 1
  // comment
  #   define A 0
  #endif
  #endif

  #ifndef HEADER_H
  #define HEADER_H
  #if 1
     // comment
  #   define A 0
  #endif
  #endif

This happens because we check vertical alignment against the '#' yet
indent to the level of the 'define'. This patch resolves this issue by
aligning against the '#'.

Reviewers: krasimir, klimek, djasper

Reviewed By: krasimir

Subscribers: cfe-commits
Differential Revision: https://reviews.llvm.org/D42408

llvm-svn: 323904