Improve "set debug separate-debug-file"

"set debug separate-debug-file" shows which candidates are considered,
when trying to find separate debug info.  But it's not clear if GDB used
a certain candidate, and if not, why not.  This patch adds some
precision:

Before:

  Looking for separate debug info (debug link) for /lib/x86_64-linux-gnu/libc.so.6
    Trying /lib/x86_64-linux-gnu/libc-2.23.so
    Trying /lib/x86_64-linux-gnu/.debug/libc-2.23.so
    Trying /usr/lib/debug//lib/x86_64-linux-gnu/libc-2.23.so

After:

  Looking for separate debug info (debug link) for /lib/x86_64-linux-gnu/libc.so.6
    Trying /lib/x86_64-linux-gnu/libc-2.23.so... no, same file as the objfile.
    Trying /lib/x86_64-linux-gnu/.debug/libc-2.23.so... no, unable to open.
    Trying /usr/lib/debug//lib/x86_64-linux-gnu/libc-2.23.so... yes!

gdb/ChangeLog:

	* build-id.c (build_id_to_debug_bfd): Enhance debug output.
	* symfile.c (separate_debug_file_exists): Likewise.

1 files changed, 27 insertions, 4 deletions

diff --git a/gdb/build-id.c b/gdb/build-id.c
index c8eacbd..ea6b61d 100644
--- a/gdb/build-id.c
+++ b/gdb/build-id.c
@@ -98,7 +98,10 @@ build_id_to_debug_bfd (size_t build_id_len, const bfd_byte *build_id)
       link += ".debug";
 
       if (separate_debug_file_debug)
-	printf_unfiltered (_("  Trying %s\n"), link.c_str ());
+	{
+	  printf_unfiltered (_("  Trying %s..."), link.c_str ());
+	  gdb_flush (gdb_stdout);
+	}
 
       /* lrealpath() is expensive even for the usually non-existent files.  */
       gdb::unique_xmalloc_ptr<char> filename;
@@ -106,16 +109,36 @@ build_id_to_debug_bfd (size_t build_id_len, const bfd_byte *build_id)
 	filename.reset (lrealpath (link.c_str ()));
 
       if (filename == NULL)
-	continue;
+	{
+	  if (separate_debug_file_debug)
+	    printf_unfiltered (_(" no, unable to compute real path\n"));
+
+	  continue;
+	}
 
       /* We expect to be silent on the non-existing files.  */
       abfd = gdb_bfd_open (filename.get (), gnutarget, -1);
 
       if (abfd == NULL)
-	continue;
+	{
+	  if (separate_debug_file_debug)
+	    printf_unfiltered (_(" no, unable to open.\n"));
+
+	  continue;
+	}
 
       if (build_id_verify (abfd.get(), build_id_len, build_id))
-	break;
+	{
+	  if (separate_debug_file_debug)
+	    printf_unfiltered (_(" yes!\n"));
+
+	  break;
+	}
+      else
+	{
+	  if (separate_debug_file_debug)
+	    printf_unfiltered (_(" no, build-id does not match.\n"));
+	}
 
       abfd.release ();
     }