[TableGen] Simplify prepSkipToLineEnd for preprocessing

The MemoryBuffer is created using `RequiresNullTerminator`, so we can
safely skip the `CurPtr != CurBuf.end()` check. The redundant check
causes a cppcheck report. In addition, elsewhere, including `*CurPtr ==
'#'` below, makes the null terminator assumption as well.

Close #81120