libstdc++: Improve ostream output for std::stacktrace

libstdc++: Improve ostream output for std::stacktrace

With this change stacktrace entries always output the frame address, and
source file information no longer results in " at :0", e.g.

  16#  myfunc(int) at /tmp/bt.cc:48 [0x4008b7]
  17#  main at /tmp/bt.cc:61 [0x40091a]
  18#  __libc_start_call_main [0x7efc3d6d3574]
  19#  __libc_start_main@GLIBC_2.2.5 [0x7efc3d6d3627]
  20#  _start [0x400684]

This replaces the previous output:

  16# myfunc(int) at /tmp/bt.cc:48
  17# main at /tmp/bt.cc:61
  18# __libc_start_call_main at :0
  19# __libc_start_main@GLIBC_2.2.5 at :0
  20# _start at :0

A change that is not visible in the examples above is that for a
non-empty stacktrace_entry, we now print "<unknown>" for the function
name if description() returns an empty string.  For an empty (e.g.
default constructed) stacktrace_entry the entire string representation
is now "<unknown>" instead of an empty string.

Instead of printing "<unknown>" for the function name, we could set that
string in the stacktrace_entry::_Info object, so that description()
returns "<unknown>" and then operator<< wouldn't need to handle an empty
description() string. However, returning an empty string from that
function seems simpler for users to detect, rather than having to parse
"<unknown>".

We could also choose a different string for an empty stacktrace_entry,
maybe "<none>" or "<invalid>", but "<unknown>" seems good.

libstdc++-v3/ChangeLog:

	* include/std/stacktrace
	(operator<<(ostream&, const stacktrace_entry&)): Improve output
	when description() or source_file() returns an empty string,
	or the stacktrace_entry is invalid. Append frame address to
	output.
	(operator<<(ostream&, const basic_stacktrace<A>&)): Use the
	size_type of the correct specialization.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Nathan Myers <nmyers@redhat.com>