[clang-format] Handle trailing comment sections in import statement lines

Summary:
This patch updates the handling of multiline trailing comment sections in
import statement lines to make it more consistent with the case in general.
This includes updating the parsing logic to collect the trailing comment
sections and the formatting logic to not insert escaped newlines at the end of
comment lines in import statement lines.

Specifically, before this patch this code:
```
#include <a> // line 1
             // line 2
```
will be turned into two unwrapped lines, whereas this code:
```
int i; // line 1
       // line 2
```
is turned into a single unwrapped line, enabling reflowing across comments.

An example where the old behaviour is bad is when partially formatting the lines
3 to 4 of this code:
```
#include <a> // line 1
             // line 2

int i;
```
which gets turned into:
```
#include <a> // line 1
             // line 2

             int i;
```
because the two comment lines were independent and the indent was copied.

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D33351

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@303415 91177308-0d34-0410-b5e6-96231b3b80d8