Re-apply 4b6c2cd642 "Deferred Concept Instantiation Implementation""

This reverts commit 0c31da48.

I've solved the issue with the PointerUnion by making the
`FunctionTemplateDecl` pointer be a NamedDecl, that could be a
`FunctionDecl` or `FunctionTemplateDecl` depending.  This is enforced
with an assert.