[MC,ELF] Emit warning if a string constant contains newline char.

GAS emits warning about newline in the string constant
so make the same behaviour.

Author: chestnykh

llvm/lib/MC/MCParser/AsmLexer.cpp

@@ -21,6 +21,7 @@
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/SMLoc.h"
 #include "llvm/Support/SaveAndRestore.h"
+#include "llvm/Support/raw_ostream.h"
 #include <cassert>
 #include <cctype>
 #include <cstdio>
@@ -646,13 +647,17 @@ AsmToken AsmLexer::LexQuote() {
     return AsmToken(AsmToken::String, StringRef(TokStart, CurPtr - TokStart));
   }
 
-  // TODO: does gas allow multiline string constants?
+  // gas doesn't allow multiline string constants
+  // and emits a warning if a string constant contains newline character
   while (CurChar != '"') {
     if (CurChar == '\\') {
       // Allow \", etc.
       CurChar = getNextChar();
     }
 
+    if (CurChar == '\n')
+      outs() << "Warning: unterminated string; newline inserted\n";
+
     if (CurChar == EOF)
       return ReturnError(TokStart, "unterminated string constant");
 

llvm/test/MC/ELF/warn-newline-in-string-constant.s

@@ -0,0 +1,6 @@
+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o %t | FileCheck %s
+
+.string "abcdefg
+12345678"
+
+// CHECK: Warning: unterminated string; newline inserted