[flang] Remove OwningPointer and ForwardReference

Use std::unique_ptr<> with custom deleter for forward-referenced owned pointer.

Move CopyableIndirection into common, add documentation, clean up.

Remove OwningPointer and ForwardReference

Use std::unique_ptr<> with custom deleter for forward-referenced owned pointer.

Use CopyableIndirection

clean up from merge after split

Complete characterization

fold conversions of arrays

Clean up subscripts to constant arrays

Elemental unary operations complete

Support assumed type TYPE(*) in actual arguments

clean up some TODOs

recognize TYPE(*) arguments to intrinsics

Complete folding of array operations

Finish elementwise array folding, add test, debug

characterize intrinsics, fix some bugs

Clean up build

Type compatibility and shape conformance checks on pointer assignments

Original-commit: flang-compiler/f18@99d734c6213c7188ad7b031258d0163826d562ec
Reviewed-on: https://github.com/flang-compiler/f18/pull/442
Tree-same-pre-rewrite: false