QmlJS: Fix line number for string literals (Part 2)

Amends 626807c94e. Scanning the strings appears differently if
using multi-line strings and using the line terminator at the beginning
of a string or having already some characters present.

Fixes: QTCREATORBUG-23777
Change-Id: I840a11e3b0e06adda443849f926189bda9dc2e4b
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@qt.io>

src/libs/qmljs/parser/qmljslexer.cpp

@@ -877,12 +877,13 @@ int Lexer::scanString(ScanStringMode mode)
     // in case we just parsed a \r, we need to reset this flag to get things working
     // correctly in the loop below and afterwards
     _skipLinefeed = false;
-
+    bool first = true;
     if (_engine) {
         while (_codePtr <= _endPtr) {
             if (isLineTerminator()) {
                 if ((quote == QLatin1Char('`') || qmlMode())) {
-                    --_currentLineNumber;
+                    if (first)
+                        --_currentLineNumber;
                     break;
                 }
                 _errorCode = IllegalCharacter;
@@ -910,6 +911,7 @@ int Lexer::scanString(ScanStringMode mode)
             // don't use scanChar() here, that would transform \r sequences and the midRef() call would create the wrong result
             _char = *_codePtr++;
             ++_currentColumnNumber;
+            first = false;
         }
     }