Make the new QStringLiteral produce a non-temporary

Make the new QStringLiteral produce a non-temporary

This is currently GCC-only code (the __extension ({ }) stuff), but it
was the only way I found to create a static variable and return it. I
had to add the QConstStringDataPtr container because I had to return a
pointer to the data -- it's impossible to return a reference through
this extension.

I could have returned a naked pointer directly too.

This isn't complete. GCC 4.6 is placing the string in the .data
section, not .rodata as we wanted. Investigating further.

Change-Id: I170030dca3eddbd69f6c879e952f652f7fe5d958
Reviewed-on: http://codereview.qt.nokia.com/1350
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>