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

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-flang-rhel-clang` running on `ppc64le-flang-rhel-test` while building `llvm` at step 7 "test-build-unified-tree-check-flang-rt".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/41925

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 7 (test-build-unified-tree-check-flang-rt) failure: 1200 seconds without output running [b'ninja', b'check-flang-rt'], attempting to kill
...
21.605 [2/8/33] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/TemporaryStack.cpp.o
21.680 [2/7/34] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/NumericalFormatTest.cpp.o
22.556 [2/6/35] Linking CXX executable flang-rt/unittests/Evaluate/reshape.test
23.345 [2/5/36] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/Format.cpp.o
23.949 [2/4/37] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/Transformational.cpp.o
24.668 [2/3/38] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/CommandTest.cpp.o
32.435 [2/2/39] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/CharacterTest.cpp.o
49.628 [2/1/40] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/Reduction.cpp.o
53.064 [1/1/41] Linking CXX executable flang-rt/unittests/Runtime/RuntimeTests
53.064 [0/1/41] Running the Flang-RT regression tests
command timed out: 1200 seconds without output running [b'ninja', b'check-flang-rt'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1262.145622

```

</details>

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