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>

libstdc++-v3/include/std/stacktrace

@@ -683,13 +683,32 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   inline ostream&
   operator<<(ostream& __os, const stacktrace_entry& __f)
   {
+    if (!__f) [[unlikely]]
+      return __os << "<unknown>";
+
     string __desc, __file;
     int __line;
-    if (__f._M_get_info(&__desc, &__file, &__line))
+    if (__f._M_get_info(&__desc, &__file, &__line)) [[likely]]
       {
-	__os.width(4);
+	__os << ' ';
-	__os << __desc << " at " << __file << ':' << __line;
+	if (__desc.empty()) [[unlikely]]
+	  __os << "<unknown>";
+	else
+	  __os << __desc;
+	if (!__file.empty()) [[likely]]
+	  __os << " at " << __file << ':' << __line;
       }
+
+    struct _Flag_guard // Set and restore hex format
+    {
+      _Flag_guard(ios& __s) : _M_ios(__s) { }
+      ~_Flag_guard() { _M_ios.setf(_M_f); }
+
+      ios& _M_ios;
+      ios::fmtflags _M_f = _M_ios.setf(ios::hex, ios::basefield);
+    };
+    _Flag_guard __g(__os);
+    __os << " [0x" << __f.native_handle() << ']';
     return __os;
   }
 
@@ -697,7 +716,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     inline ostream&
     operator<<(ostream& __os, const basic_stacktrace<_Allocator>& __st)
     {
-      for (stacktrace::size_type __i = 0; __i < __st.size(); ++__i)
+      using size_type = typename basic_stacktrace<_Allocator>::size_type;
+      for (size_type __i = 0, __size = __st.size(); __i < __size; ++__i)
 	{
 	  __os.width(4);
 	  __os << __i << "# " << __st[__i] << '\n';