[gdb] Fix range loop index in find_method

With target board debug-types, we have:
...
FAIL: gdb.cp/cpexprs.exp: list policy1::function
...

This is a regression triggered by commit 770479f223e "gdb: Fix toplevel types
with -fdebug-types-section".

However, the FAIL is caused by commit 4dedf84da98 "Change
decode_compound_collector to use std::vector" which changes a VEC_iterate loop
into a range loop:
...
-  for (ix = 0; VEC_iterate (symbolp, sym_classes, ix, sym); ++ix)
+  unsigned int ix = 0;
+  for (const auto &sym : *sym_classes)
...
but fails to ensure that the increment of ix happens every iteration.

Fix this by calculating the index variable at the start of the loop body:
...
  for (const auto &elt : *sym_classes)
    {
      unsigned int ix = &elt - &*sym_classes->begin ();
...

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

gdb/ChangeLog:

2020-04-29  Tom de Vries  <tdevries@suse.de>

	PR symtab/25889
	* linespec.c (find_method): Fix ix calculation.

gdb/testsuite/ChangeLog:

2020-04-29  Tom de Vries  <tdevries@suse.de>

	PR symtab/25889
	* gdb.cp/cpexprs.exp: Adapt for inclusion.
	* gdb.cp/cpexprs-debug-types.exp: New file.  Set -fdebug-types-section
	and include cpexprs.exp.

---
 gdb/linespec.c                               |  3 +--
 gdb/testsuite/gdb.cp/cpexprs-debug-types.exp | 20 ++++++++++++++++++++
 gdb/testsuite/gdb.cp/cpexprs.exp             | 14 ++++++++++++--
 5 files changed, 45 insertions(+), 4 deletions(-)

Index: gdb-9.1/gdb/linespec.c
===================================================================
--- gdb-9.1.orig/gdb/linespec.c
+++ gdb-9.1/gdb/linespec.c
@@ -3670,12 +3670,12 @@ find_method (struct linespec_state *self
      because we collect data across the program space before deciding
      what to do.  */
   last_result_len = 0;
-  unsigned int ix = 0;
   for (const auto &elt : *sym_classes)
     {
       struct type *t;
       struct program_space *pspace;
       struct symbol *sym = elt.symbol;
+      unsigned int ix = &elt - &*sym_classes->begin ();
 
       /* Program spaces that are executing startup should have
 	 been filtered out earlier.  */
@@ -3706,7 +3706,6 @@ find_method (struct linespec_state *self
 
 	  superclass_vec.clear ();
 	  last_result_len = result_names.size ();
-	  ++ix;
 	}
     }
 
Index: gdb-9.1/gdb/testsuite/gdb.cp/cpexprs-debug-types.exp
===================================================================
--- /dev/null
+++ gdb-9.1/gdb/testsuite/gdb.cp/cpexprs-debug-types.exp
@@ -0,0 +1,20 @@
+# Copyright 2020 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This file is part of the gdb testsuite.
+
+# Run cpexprs.exp with -fdebug-types-section.
+set flags {additional_flags=-fdebug-types-section}
+source $srcdir/$subdir/cpexprs.exp
Index: gdb-9.1/gdb/testsuite/gdb.cp/cpexprs.exp
===================================================================
--- gdb-9.1.orig/gdb/testsuite/gdb.cp/cpexprs.exp
+++ gdb-9.1/gdb/testsuite/gdb.cp/cpexprs.exp
@@ -685,13 +685,23 @@ if {[skip_cplus_tests]} { continue }
 # test running programs
 #
 
-standard_testfile .cc
+standard_testfile cpexprs.cc
 
 if {[get_compiler_info "c++"]} {
     return -1
 }
 
-if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug c++}]} {
+if { [info exists flags] } {
+    # Already set externally.
+} else {
+    # Initialize to empty.
+    set flags {}
+}
+
+# Include required flags.
+set flags "$flags debug c++"
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile "$flags"]} {
     return -1
 }