c++: Fix thinko in enum_min_precision [PR61414]

c++: Fix thinko in enum_min_precision [PR61414]

When backporting the PR61414 fix to 8.4, I've noticed that the caching
of prec is actually broken, as it would fail to actually store the computed
precision into the hash_map's value and so next time we'd think the enum needs
0 bits.

2020-02-14  Jakub Jelinek  <jakub@redhat.com>

	PR c++/61414
	* class.c (enum_min_precision): Change prec type from int to int &.

	* g++.dg/cpp0x/enum39.C: New test.