[clang-format] Fix comment aligning when there are changes within the comment

As soon as a comment had whitespace changes inside of the token, we
couldn't identify the whole comment as a trailing comment anymore and
alignment stopped working. Add a new boolean to Change for this special
case and fix trailing comment identification to use it.

This also changes WhitespaceManager to sum the length of all Changes
inside of a token into the first Change.

Before this fix

  int xy;  // a
  int z;   //b

became

  int xy;  // a
  int z;  // b

with this patch we immediately get to:

  int xy;  // a
  int z;   // b

Differential Revision: http://reviews.llvm.org/D16058

llvm-svn: 257341