[SemaTemplate] Stop passing insertion position around during VarTemplate instantiation

They can get stale at use time because of updates from other recursive
specializations. Instead, rely on the existence of previous declarations to add
the specialization.

Differential Revision: https://reviews.llvm.org/D87853

(cherry picked from commit cffb0dd5)

This is a re-commit of 8ac70957 with
some modifications to avoid changing the clang API.