[clang-format] Keep trailing preprocessor line comments separate from the...

[clang-format] Keep trailing preprocessor line comments separate from the following section comments

Summary:
r303415 changed the way a sequence of line comments following a preprocessor
macro is handled, which has the unfortunate effect of aligning a trailing
preprocessor line comment and following unrelated section comments, so:
```
#ifdef A // comment about A
// section comment
#endif
```
gets turned into:
```
#ifdef A // comment about A
         // section comment
#endif
```
This patch fixes this by additionally checking the original start columns of
the line comments.

Reviewers: djasper

Reviewed By: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D33394

llvm-svn: 303541