[gdb/symtab] Fix line number of static const class member

Since commit 6d263fe46e ("Avoid bad breakpoints with --gc-sections"), there
was a silent regression on openSUSE Leap 15.4 for test-case
gdb.cp/m-static.exp, from:
...
(gdb) info variable everywhere^M
All variables matching regular expression "everywhere":^M
^M
File /home/vries/tmp.local-remote-host-native/m-static.h:^M
8:      const int gnu_obj_4::everywhere;^M
(gdb)
...
to:
...
(gdb) info variable everywhere^M
All variables matching regular expression "everywhere":^M
^M
File /data/vries/gdb/src/gdb/testsuite/gdb.cp/m-static.h:^M
8:      const int gnu_obj_4::everywhere;^M
^M
File /data/vries/gdb/src/gdb/testsuite/gdb.cp/m-static1.cc:^M
8:      const int gnu_obj_4::everywhere;^M
(gdb)
...

Another regression was found due to that commit, and it was fixed in commit
99d679e7b3 ("[gdb/symtab] Fix "file index out of range" complaint") by
limiting the scope of the fix in the original commit.

Fix this regression by yet further limiting the scope of that fix, making sure
that this bit in dwarf_decode_lines is executed again for m-static1.cc:
...
  /* Make sure a symtab is created for every file, even files
     which contain only variables (i.e. no code with associated
     line numbers).  */
...

Tested on x86_64-linux.

PR symtab/30265
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30265

gdb/dwarf2/read.c

@@ -7610,8 +7610,7 @@ handle_DW_AT_stmt_list (struct die_info *die, struct dwarf2_cu *cu,
      then there won't be any interesting code in the CU, but a check later on
      (in lnp_state_machine::check_line_address) will fail to properly exclude
      an entry that was removed via --gc-sections.  */
-  if (have_code)
-    dwarf_decode_lines (cu->line_header, cu, lowpc, decode_mapping);
+  dwarf_decode_lines (cu->line_header, cu, lowpc, decode_mapping && have_code);
 }
 
 /* Process DW_TAG_compile_unit or DW_TAG_partial_unit.  */

gdb/testsuite/gdb.cp/m-static.exp

@@ -185,8 +185,10 @@ gdb_test "print test4.somewhere" "\\$\[0-9\].* = 3.14\[0-9\]*" "static const flo
 if { $non_dwarf } { setup_xfail *-*-* }
 gdb_test "info variable everywhere" \
     [multi_line \
+	 {All variables matching regular expression "everywhere":} \
+	 "" \
 	 "File (.*/)?m-static\[.\]h:" \
-	 "$decimal:\tconst int gnu_obj_4::everywhere;.*"]
+	 "$decimal:\tconst int gnu_obj_4::everywhere;"]
 
 # Perhaps at some point test4 should also include a test for a static
 # const int that was initialized in the header file.  But I'm not sure