Fix buglet in cp_print_value_fields patch

[ Backport of master commit 3d507ff23b. ]

Pedro pointed out an issue in the cp_print_value_fields
patch, aka the fix for PR c++/20020.

This patch addresses the issue.  Tested on x86-64 Fedora 29.

gdb/testsuite/ChangeLog
2019-06-27  Tom Tromey  <tromey@adacore.com>

	* gdb.cp/constexpr-field.exp: Use setup_xfail.

---
 gdb/testsuite/ChangeLog                  | 4 ++++
 gdb/testsuite/gdb.cp/constexpr-field.exp | 3 ++-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 5f26daa67e..c51036165c 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,7 @@
+2019-06-27  Tom Tromey  <tromey@adacore.com>
+
+	* gdb.cp/constexpr-field.exp: Use setup_xfail.
+
 2019-05-29  Tom Tromey  <tromey@adacore.com>
 
 	PR c++/20020:
diff --git a/gdb/testsuite/gdb.cp/constexpr-field.exp b/gdb/testsuite/gdb.cp/constexpr-field.exp
index e4849efeae..2f71cef952 100644
--- a/gdb/testsuite/gdb.cp/constexpr-field.exp
+++ b/gdb/testsuite/gdb.cp/constexpr-field.exp
@@ -31,6 +31,7 @@ if {![runto_main]} {
 
 # "x" sometimes isn't available due to
 # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90526
-gdb_test "print x" " = {static f = .*}"
+setup_xfail *-*-* gcc/90526
+gdb_test "print x" " = {static f = true}"
 
 gdb_test "print y" " = {static f = true}"