[clang-format] Fix regression about not aligning trailing comments in case they were previously aligned, but at different indent.

Summary:
Comment reflower was adding untouchable tokens in case two consecutive comment lines are aligned in the source code. This disallows the whitespace manager to re-indent them later.

source:
```
int i = f(abc, // line 1
          d, // line 2
	     // line 3
	  b);
```
Since line 2 and line 3 are aligned, the reflower was marking line 3 as untouchable; however the three comment lines need to be re-aligned.
output before:
```
int i = f(abc, // line 1
          d,   // line 2
	     // line 3
	  b);
```
output after:
```
int i = f(abc, // line 1
          d,   // line 2
	       // line 3
	  b);
```

Reviewers: djasper

Reviewed By: djasper

Subscribers: sammccall, cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D29383

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@293755 91177308-0d34-0410-b5e6-96231b3b80d8