[MC] Fix ICE with non-newline terminated input

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.

Reviewed By: niravd

Differential Revision: https://reviews.llvm.org/D92681

llvm/lib/MC/MCParser/AsmLexer.cpp

@@ -715,7 +715,7 @@ AsmToken AsmLexer::LexToken() {
   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;

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

@@ -0,0 +1 @@
+.text

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

@@ -1,5 +1,6 @@
 	#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
@@ -11,3 +12,6 @@ foo:	#Comment here
 	#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