[MC] Fix ICE with non-newline terminated input

[scott.linder]

Summary

There is an explicit option for the lexer to support this, but we crash
when -preserve-comments is enabled because it checks for
getTok().getString().empty() to detect the case. This doesn't
work currently because the lexer reports this case as a string of length
1, containing a null byte.

Change the lexer to instead report this case via an empty string, as the
null terminator isn't logically a part of the textual input, and the
check for .empty() seems natural and obvious in the calling code.

[scott.linder]

Alternatively I'm open to fix this in the caller if the (EndOfStatement, "\0") value is correct here. I just didn't see what use the caller would have for a string containing an embedded null here.

[niravd]

LGTM.

llvm/lib/MC/MCParser/AsmLexer.cpp

return AsmToken(AsmToken::EndOfStatement,
                     StringRef(TokStart, strlen(MAI.getSeparatorString())));
   }
 
   // If we're missing a newline at EOF, make sure we still get an
   // EndOfStatement token before the Eof token.
   if (CurChar == EOF && !IsAtStartOfStatement && EndStatementAtEOF) {
     IsAtStartOfLine = true;
     IsAtStartOfStatement = true;
-    return AsmToken(AsmToken::EndOfStatement, StringRef(TokStart, 1));
+    return AsmToken(AsmToken::EndOfStatement, StringRef(TokStart, 0));
   }
   IsAtStartOfLine = false;
   bool OldIsAtStartOfStatement = IsAtStartOfStatement;
   IsAtStartOfStatement = false;
   switch (CurChar) {
   default:
     if (MAI.doesAllowSymbolAtNameStart()) {
       // Handle Microsoft-style identifier: [a-zA-Z_$.@?][a-zA-Z0-9_$.@?]*

llvm/test/MC/AsmParser/Inputs/no-newline-at-end-of-file.s

+.text
 No newline at end of file

llvm/test/MC/AsmParser/preserve-comments.s

   #RUN: llvm-mc -preserve-comments -n -triple i386-linux-gnu < %s > %t
   #RUN: diff -b %s %t
+  #RUN: llvm-mc -preserve-comments -n -triple i386-linux-gnu < %p/Inputs/no-newline-at-end-of-file.s | FileCheck %s
   .text
 
 foo:  #Comment here
   #comment here
   nop
   #if DIRECTIVE COMMENT
   ## WHOLE LINE COMMENT
   cmpl  $196, %eax  ## EOL COMMENT
   #endif
   .ident  "clang version 3.9.0"
   .section  ".note.GNU-stack","",@progbits
+
+  #Confirm we don't crash on inputs without a terminating newline.
+  #CHECK: .text