[gdb/exp] Fix printing of type of optimized out vla

Consider this snippet from gcc/testsuite/gcc.dg/guality/vla-1.c:
...
int __attribute__((noinline))
f1 (int i)
{
  char a[i + 1];
  a[0] = 5;             /* { dg-final { gdb-test .+1 "i" "5" } } */
  return a[0];          /* { dg-final { gdb-test . "sizeof (a)" "6" } } */
}
...

When we compile the test-case with -O1 -g, and query the size of optimized
out vla 'a', we get:
...
$ ./gdb -batch -ex "b f1" -ex "r" -ex "p sizeof (a)" vla-1.exe
Breakpoint 1 at 0x4004a8: file vla-1.c, line 17.

Breakpoint 1, f1 (i=i@entry=5) at vla-1.c:17
17        return a[0];
$1 = 0
...
while we expect a size of '6'.

The problem is that default_read_var_value does not resolve the dynamic type
of a variable if the variable is optimized out.

This patch fixes that, and consequently gdb prints '6', as expected.

Tested on x86_64-linux.

2018-07-18  Tom de Vries  <tdevries@suse.de>

	* findvar.c (default_read_var_value): Also resolve dynamic type for
	LOC_OPTIMIZED_OUT vars.

	* gdb.base/vla-optimized-out.c: New test.
	* gdb.base/vla-optimized-out.exp: New file.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2018-07-18  Tom de Vries  <tdevries@suse.de>
+
+	* findvar.c (default_read_var_value): Also resolve dynamic type for
+	LOC_OPTIMIZED_OUT vars.
+
 2018-07-18  Maciej W. Rozycki  <macro@mips.com>
 
 	* mips-tdep.c (micromips_next_pc): Add SYSCALL instruction

gdb/findvar.c

@@ -789,6 +789,8 @@ default_read_var_value (struct symbol *var, const struct block *var_block,
       break;
 
     case LOC_OPTIMIZED_OUT:
+      if (is_dynamic_type (type))
+	type = resolve_dynamic_type (type, NULL, /* Unused address.  */ 0);
       return allocate_optimized_out_value (type);
 
     default:

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2018-07-18  Tom de Vries  <tdevries@suse.de>
+
+	* gdb.base/vla-optimized-out.c: New test.
+	* gdb.base/vla-optimized-out.exp: New file.
+
 2018-07-13  Sergio Durigan Junior  <sergiodj@redhat.com>
 
 	* lib/gdbserver-support.exp (gdbserver_start): Expect for the

gdb/testsuite/gdb.base/vla-optimized-out.c

@@ -0,0 +1,34 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2018 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/>.  */
+
+int __attribute__((noinline, noclone))
+f1 (int i)
+{
+  char a[i + 1];
+  a[0] = 5;
+  return a[0];
+}
+
+int
+main (void)
+{
+  volatile int j;
+  int i = 5;
+  asm volatile ("" : "=r" (i) : "0" (i));
+  j = f1 (i);
+  return 0;
+}

gdb/testsuite/gdb.base/vla-optimized-out.exp

@@ -0,0 +1,40 @@
+# Copyright 2018 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/>.
+
+# Check whether we can determine the size of an optimized-out vla.
+
+standard_testfile
+
+if { [prepare_for_testing "failed to prepare" $testfile $srcfile \
+	  {debug optimize=-O1}] } {
+    return -1
+}
+
+proc vla_optimized_out { } {
+    if ![runto f1] {
+	fail "can't run to f1"
+	return
+    }
+
+    gdb_test "p a" \
+	{ = <optimized out>} \
+	"printed optimized out vla"
+
+    gdb_test "p sizeof (a)" \
+	{ = 6} \
+	"printed size of optimized out vla"
+}
+
+vla_optimized_out