There is no debug info when the DECL_IGNORED_P flag
is set. But sometimes we have the line info of the
function decl, as in the case of on virtual thunks.
So instead of no line info at all, we emit at least
the location of the function decl.
On the other side, there are DECL_IGNORED_P functions
which do not have any source line info at all.
Remove those from the debug_range info, to make it
clear for the debugger that the line info for these
functions is invalid. This has the effect that the
debugger will not step into the function without
debug info.