[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