Improve lexer error recovery with interpolated strings.

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6794,4 +6794,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_SimpleProgramIsEmpty" xml:space="preserve">
     <value>At least one top-level statement must be non-empty.</value>
   </data>
-</root>
+  <data name="ERR_NewlinesAreNotAllowedInsideANonVerbatimInterpolatedString" xml:space="preserve">
+    <value>Newlines are not allowed inside a non-verbatim interpolated string</value>
+  </data>
+</root>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -1981,6 +1981,7 @@ internal enum ErrorCode
         ERR_FileScopedAndNormalNamespace = 8955,
         ERR_FileScopedNamespaceNotBeforeAllMembers = 8956,
         ERR_NoImplicitConvTargetTypedConditional = 8957,
+        ERR_NewlinesAreNotAllowedInsideANonVerbatimInterpolatedString = 8959,
 
         #endregion
 

src/Compilers/CSharp/Portable/Parser/Lexer.cs

@@ -764,7 +764,9 @@ private void ScanSyntaxToken(ref TokenInfo info)
                 case '@':
                     if (TextWindow.PeekChar(1) == '"')
                     {
-                        this.ScanVerbatimStringLiteral(ref info);
+                        var errorCode = this.ScanVerbatimStringLiteral(ref info, allowNewlines: true);
+                        if (errorCode is ErrorCode code)
+                            this.AddError(code);
                     }
                     else if (TextWindow.PeekChar(1) == '$' && TextWindow.PeekChar(2) == '"')
                     {