[llvm] [MC] Fix accidentally eating the newline when handling a comment char at the end of the line. (PR #165129)

[Amara Emerson via llvm-commits]

aemerson wrote:

### Merge activity

* **Oct 26, 5:49 AM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/165129).


https://github.com/llvm/llvm-project/pull/165129