Revisit PR10177: don't instantiate a variable if it's only referenced in a

Revisit PR10177: don't instantiate a variable if it's only referenced in a
dependent context and can't be used in a constant expression.

Per C++ [temp.inst]p2, "the instantiation of a static data member does not
occur unless the static data member is used in a way that requires the
definition to exist".

This doesn't /quite/ match that, as we still instantiate static data members
that are usable in constant expressions even if the use doesn't require a
definition. A followup patch will fix that for both variables and functions.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@291295 91177308-0d34-0410-b5e6-96231b3b80d8