2bbe3efd1¹ added the -Wdeprecated-copy warning, and fixed the instances
of it which GCC warned about, in HintFmt and ref<T>. However, when
building with Clang, there is an additional deprecated-copy warning in
BaseError. This commit explicitly defaults the copy assignment operator
for BaseError and silences this warning.
1: 2bbe3efd16
Change-Id: I50aa4a7ab1a7aae5d7b31f765994abd3db06379d