Summary:
QCoreApplication::addLibraryPath is called before QCoreApplication
was constructed and this caused the returned string to be different
depending from which working directory it was called and not always
the intended binary path.
Using qt.conf has a fixed prefix relative to the binary path
inside the application bundle and therefore is easier to be used
with a relative path.
Reviewers: kbasan, msutcliffe
Reviewed By: msutcliffe
Differential Revision: https://dev.swift-project.org/D18
