[Macros] Adjust insertion position of peer macros for var decls
When the target decl is a VarDecl, peer macro should expand at the end
of the pattern binding decl, instead of the end of the VarDecl which
only covers the name.
rdar://117098598
(cherry picked from commit ac1443ada6a7e750a02e10f4eb6ec191a013afe1)
