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>