[gdb] Fix range loop index in find_method

[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.