[gdb] Notice when stepping into different fileHEADmaster

Consider the following test-case:
...
$ cat -n test.c
     1	int var;
     2
     3	int
     4	foo (void)
     5	{
     6	  var = 1;
     7	#include "test.h"
     8	}
     9
    10	int
    11	main ()
    12	{
    13	  return foo ();
    14	}
$ cat -n test.h
     1	  return 1;
$ gcc test.c -g
...

When stepping through the test-case, gdb doesn't make it explicit that line 1
is not in test.c:
...
Temporary breakpoint 1, main () at test.c:13
13	  return foo ();
(gdb) step
foo () at test.c:6
6	  var = 1;
(gdb) n
1	  return 1;
(gdb)
8	}
(gdb)
...
which makes it easy to misinterpret the output.

This is with the default "print frame-info" == auto, with documented
behaviour [1]:
...
stepi will switch between source-line and source-and-location depending on the
program counter.
...

What is actually implemented is that source-line is used unless stepping into
or out of a function.

The problem can be worked around by using
"set print frame-info source-and-location", but that's a bit verbose.

Instead, change the behaviour of "print frame-info" == auto to also use
source-and-location when stepping into another file, which gets us:
...
(gdb) n
foo () at test.h:1
1	  return 1;
...

Tested on x86_64-linux.

Reviewed-By: Kevin Buettner <kevinb@redhat.com>
Reviewed-By: Kévin Le Gouguec <legouguec@adacore.com>

PR gdb/32011
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32011

[1] https://sourceware.org/gdb/current/onlinedocs/gdb.html/Print-Settings.html#index-set-print-frame_002dinfo

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