[gdb/ada] Fix -readnow FAILs

When running test-case gdb.ada/access_to_packed_array we have:
...
(gdb) print pack.a^M
$1 = (0 => 1, 2, 3, 4, 5, 6, 7, 8, 9, 10)^M
...
but with target board readnow.exp, we have instead:
...
(gdb) print pack.a^M
'pack.a' has unknown type; cast it to its declared type^M
...

The symbol is normally found by the map_matching_symbols call in
ada-lang.c:add_nonlocal_symbols:
...
  for (objfile *objfile : current_program_space->objfiles ())
    {
      data.objfile = objfile;

      objfile->sf->qf->map_matching_symbols (objfile, lookup_name,
					     domain, global, callback,
					     (is_wild_match
					      ? NULL : compare_names));
...
which maps onto psym_map_matching_symbols.

Function psym_map_matching_symbols iterates over all the partial symtabs,
and:
- if not expanded, searches in the partial symtab:
  - if not found, continues to the next
  - if found, expands into full symtab
- searches in the full symtab

However, with -readnow the call maps onto dw2_map_matching_symbols instead,
which is unimplemented, and consequently no symbol is found.

Fix this by detecting -readnow in dw2_map_matching_symbols, and handling that
appropriately given that partial symtabs are not present, and full symtabs
are: iterate over all the symtabs and search them.

Tested on x86_64-linux, with native and target board -readnow.

This removes 217 FAILs with board -readnow.

gdb/ChangeLog:

2020-04-02  Tom de Vries  <tdevries@suse.de>

	PR ada/24671
	* dwarf2/read.c (dw2_map_matching_symbols): Handle -readnow.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2020-04-02  Tom de Vries  <tdevries@suse.de>
+
+	PR ada/24671
+	* dwarf2/read.c (dw2_map_matching_symbols): Handle -readnow.
+
 2020-04-02  Tom de Vries  <tdevries@suse.de>
 
 	* dwarf2/read.c (dwarf2_gdb_index_functions,

gdb/dwarf2/read.c

@@ -3592,9 +3592,35 @@ dw2_map_matching_symbols
    gdb::function_view<symbol_found_callback_ftype> callback,
    symbol_compare_ftype *ordered_compare)
 {
-  /* Currently unimplemented; used for Ada.  The function can be called if the
-     current language is Ada for a non-Ada objfile using GNU index.  As Ada
-     does not look for non-Ada symbols this function should just return.  */
+  /* Used for Ada.  */
+  struct dwarf2_per_objfile *dwarf2_per_objfile
+    = get_dwarf2_per_objfile (objfile);
+
+  if (dwarf2_per_objfile->index_table != nullptr)
+    {
+      /* Ada currently doesn't support .gdb_index (see PR24713).  We can get
+	 here though if the current language is Ada for a non-Ada objfile
+	 using GNU index.  As Ada does not look for non-Ada symbols this
+	 function should just return.  */
+      return;
+    }
+
+  /* We have -readnow: no .gdb_index, but no partial symtabs either.  So,
+     inline psym_map_matching_symbols here, assuming all partial symtabs have
+     been read in.  */
+  const int block_kind = global ? GLOBAL_BLOCK : STATIC_BLOCK;
+
+  for (compunit_symtab *cust : objfile->compunits ())
+    {
+      const struct block *block;
+
+      if (cust == NULL)
+	continue;
+      block = BLOCKVECTOR_BLOCK (COMPUNIT_BLOCKVECTOR (cust), block_kind);
+      if (!iterate_over_symbols_terminated (block, name,
+					    domain, callback))
+	return;
+    }
 }
 
 /* Starting from a search name, return the string that finds the upper