Fix crash in cp_print_value_fields

Fix crash in cp_print_value_fields

PR c++/20020 concerns a crash in cp_print_value_fields.  The immediate
cause is that cp_print_value_fields does not handle the case where
value_static_field fails.  This is fixed in this patch by calling
cp_print_static_field from the "try" block.

Digging a bit deeper, the error occurs because GCC does not emit a
DW_AT_const_value for a static constexpr member appearing in a
template class.  I've filed a GCC bug for this.

Tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-05-29  Tom Tromey  <tromey@adacore.com>

	PR c++/20020:
	* cp-valprint.c (cp_print_value_fields): Call
	cp_print_static_field inside "try".

gdb/testsuite/ChangeLog
2019-05-29  Tom Tromey  <tromey@adacore.com>

	PR c++/20020:
	* gdb.cp/constexpr-field.exp: New file.
	* gdb.cp/constexpr-field.cc: New file.