Handle missing newline at EOF more gracefully in MC AsmLexer.

Handle missing newline at EOF more gracefully in MC AsmLexer.

If we see an EOF w/o a preceding end-of-line, return an EndOfStatement
token before returning the Eof token.

Based on patch by Stepan Dyatkovskiy.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@139798 91177308-0d34-0410-b5e6-96231b3b80d8