[flang] Support for type-bound operators and assignment

Add `ArgumentAnalyzer::FindBoundOp` to look for an operator or
assignment definition in the type of each operand. Then `TryBoundOp`
checks if it is actually applicable.

Change ResolveGeneric to handle type-bound operators: the `adjustActuals`
function passed in handles the difference between these and normal
type-bound procedures. For operators, either operand may be the passed-
object argument. For procedures we know which one it is.

Extract `GetDerivedTypeSpec`, `GetBindingResolution`, and
`OkLogicalIntegerAssignment` into separate functions to simplify the
logic of the calling functions.

Original-commit: flang-compiler/f18@1f7ff22145ef61c4c168c466f4de4fab11e0f611
Reviewed-on: https://github.com/flang-compiler/f18/pull/872
Tree-same-pre-rewrite: false