If we see an explicit instantiation declaration or definition of a function
after we've already instantiated a definition for the function, pass it to the
ASTConsumer again so that it knows the specialization kind has changed and can
update the function's linkage.

This only matters if we instantiate the definition of the function before we
reach the end of the TU; this can happen in at least three different ways:
C++11 constexpr functions, C++14 deduced return types, and functions
instantiated within modules.