[gdb/symtab] Fix gdb.base/vla-optimized-out.exp with clang

Consider test-case gdb.base/vla-optimized-out.exp, compiled using clang-10.

GDB fails to get the size of the vla a:
...
(gdb) p sizeof (a)^M
Cannot access memory at address 0x6^M
(gdb) FAIL: gdb.base/vla-optimized-out.exp: o1: printed size of \
  optimized out vla
...

The relevant DWARF looks like this: the variable a:
...
 <2><12b>: Abbrev Number: 5 (DW_TAG_variable)
    <12c>   DW_AT_name        : a
    <132>   DW_AT_type        : <0x189>
...
has type:
...
 <1><189>: Abbrev Number: 10 (DW_TAG_array_type)
    <18a>   DW_AT_type        : <0x198>
 <2><18e>: Abbrev Number: 11 (DW_TAG_subrange_type)
    <18f>   DW_AT_type        : <0x19f>
    <193>   DW_AT_count       : <0x117>
...
with the count attribute equated to the value of this artificial variable:
...
 <2><117>: Abbrev Number: 4 (DW_TAG_variable)
    <118>   DW_AT_location    : 10 byte block: 75 1 10 ff ff ff ff f 1a 9f \
              (DW_OP_breg5 (rdi): 1;
	       DW_OP_constu: 4294967295;
	       DW_OP_and;
	       DW_OP_stack_value)
    <123>   DW_AT_name        : __vla_expr0
    <127>   DW_AT_type        : <0x182>
    <12b>   DW_AT_artificial  : 1
...

The location description of the variable is terminated with DW_OP_stack_value,
which according to the DWARF spec means that "the DWARF expression represents
the actual value of the object, rather than its location".

However, in attr_to_dynamic_prop, we set is_reference to true:
...
               baton->locexpr.is_reference = true;
...
and use it in dwarf2_evaluate_property to dereference the value of the DWARF
expression, which causes the access to memory at address 0x6.

Fix this by ignoring the baton->locexpr.is_reference == true setting if
the expression evaluation has ctx.location == DWARF_VALUE_STACK, such that we
get:
...
(gdb) p sizeof (a)^M
$2 = 6^M
(gdb) PASS: gdb.base/vla-optimized-out.exp: o1: printed size of \
  optimized out vla
...

Tested on x86_64-linux, with gcc.

Tested the following test-cases (the ones mentioned in PR26905) on
x86_64-linux with clang-10:
- gdb.base/vla-optimized-out.exp
- gdb.base/vla-ptr.exp
- gdb.mi/mi-vla-c99

gdb/ChangeLog:

2020-11-30  Tom de Vries  <tdevries@suse.de>

	PR symtab/26905
	* dwarf2/loc.c (dwarf2_locexpr_baton_eval): Add and handle
	is_reference parameter.
	(dwarf2_evaluate_property): Update dwarf2_locexpr_baton_eval call.

gdb/testsuite/ChangeLog:

2020-11-30  Tom de Vries  <tdevries@suse.de>

	PR symtab/26905
	* gdb.dwarf2/count.exp: Remove kfails.

1 files changed, 9 insertions, 4 deletions

diff --git a/gdb/dwarf2/loc.c b/gdb/dwarf2/loc.c
index 86d0ca4..fea49c3 100644
--- a/gdb/dwarf2/loc.c
+++ b/gdb/dwarf2/loc.c
@@ -2499,7 +2499,8 @@ dwarf2_locexpr_baton_eval (const struct dwarf2_locexpr_baton *dlbaton,
 			   struct frame_info *frame,
 			   const struct property_addr_info *addr_stack,
 			   CORE_ADDR *valp,
-			   bool push_initial_value)
+			   bool push_initial_value,
+			   bool *is_reference)
 {
   if (dlbaton == NULL || dlbaton->size == 0)
     return 0;
@@ -2546,9 +2547,12 @@ dwarf2_locexpr_baton_eval (const struct dwarf2_locexpr_baton *dlbaton,
 
   switch (ctx.location)
     {
+    case DWARF_VALUE_STACK:
+      *is_reference = false;
+      /* FALLTHROUGH */
+
     case DWARF_VALUE_REGISTER:
     case DWARF_VALUE_MEMORY:
-    case DWARF_VALUE_STACK:
       *valp = ctx.fetch_address (0);
       if (ctx.location == DWARF_VALUE_REGISTER)
 	*valp = ctx.read_addr_from_reg (*valp);
@@ -2589,10 +2593,11 @@ dwarf2_evaluate_property (const struct dynamic_prop *prop,
 	  = (const struct dwarf2_property_baton *) prop->baton ();
 	gdb_assert (baton->property_type != NULL);
 
+	bool is_reference = baton->locexpr.is_reference;
 	if (dwarf2_locexpr_baton_eval (&baton->locexpr, frame, addr_stack,
-				       value, push_initial_value))
+				       value, push_initial_value, &is_reference))
 	  {
-	    if (baton->locexpr.is_reference)
+	    if (is_reference)
 	      {
 		struct value *val = value_at (baton->property_type, *value);
 		*value = value_as_address (val);