qdoc: Print absolute path for errors and warnings

QDir::isAbsolutePath() returns true for paths that begin with root dir
or drive letter (on Windows). Since Clang integration, we now see
warnings with paths such as

/path/to/qt5/qtbase/include/QtQuick/../../../qtdeclarative/src/ [...]

Remove the check for isAbsolutePath() and always construct an absolute
path.

Change-Id: Iea1e6f83f452abf0644968e0a5ee725829a8716e
Reviewed-by: Martin Smith <martin.smith@qt.io>

src/qdoc/location.cpp

@@ -463,11 +463,8 @@ QString Location::toString() const
 
 QString Location::top() const
 {
-    QString str = filePath();
-    if (!QDir::isAbsolutePath(str)) {
-        QDir path(str);
-        str = path.absolutePath();
-    }
+    QDir path(filePath());
+    QString str = path.absolutePath();
     if (lineNo() >= 1) {
         str += QLatin1Char(':');
         str += QString::number(lineNo());