[flang][NFC] Refactor derived type instantiation

Summary:
Move InstantiateComponent and InstantiateIntrinsicType from symbol.cpp
to type.cpp as that is where they are called.

Put both in InstantiateHelper class to better isolate them.
Add CreateDerivedTypeSpec in InstantiateHelper.

Add non-const forms for AsIntrinsic and AsDerived to avoid const_cast.

No functional changes.

Reviewers: DavidTruby, klausler, PeteSteinfeld, jdoerfert

Reviewed By: klausler

Subscribers: llvm-commits

Tags: #flang, #llvm

Differential Revision: https://reviews.llvm.org/D78678