Full paths in DAP stackTrace responses

Full paths in DAP stackTrace responses

Vladimir Makaev noticed that, in some cases, a DAP stackTrace response
would include a relative path name for the "path" component.

This patch changes the frame decorator code to add a new DAP-specific
decorator, and changes the DAP entry point to frame filters to use it.
This decorator prefers the symtab's full name, and does not fall back
to the solib's name.

I'm not entirely happy with this patch, because if a user frame filter
uses FrameDecorator, it may still do the wrong thing.  It would be
better to have frame filters return symtab-like objects instead, or to
have a separate method to return the full path to the source file.

I also tend to think that the solib fallback behavior of
FrameDecorator is a mistake.  If this is ever needed, it seems to me
that it should be a separate method.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30665