Add missing diagnostic for explicit instantiation declarations naming

internal linkage entities.

Such constructs are ill-formed by [temp.explicit]p13. We make a special
exception to permit an invalid construct used by libc++ in some build
modes: its <valarray> header declares some functions with the
internal_linkage attribute and then (meaninglessly) provides explicit
instantiation declarations for them. Luckily, Clang happens to
effectively ignore the explicit instantiation declaration when
generating code in this case, and this change codifies that behavior.

This reinstates part of r359048, reverted in r359076. (The libc++ issue
triggering the rollback has been addressed.)

llvm-svn: 359259