Support explicit instantiation of function templates and members of class

templates when only the declaration is in scope. This requires deferring the
instantiation to be lazy, and ensuring the definition is required for that
translation unit. We re-use the existing pending instantiation queue,
previously only used to track implicit instantiations which were required to be
lazy. Fixes PR7979.

A subsequent change will rename *PendingImplicitInstantiations to
*PendingInstatiations for clarity given its broader role.

llvm-svn: 112037