[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@1f7ff22
Reviewed-on: flang-compiler/f18#872
Tree-same-pre-rewrite: false

flang/lib/parser/message.h

@@ -298,6 +298,10 @@ class ContextualMessages {
   common::Restorer<Messages *> SetMessages(Messages &buffer) {
     return common::ScopedSet(messages_, &buffer);
   }
+  // Discard messages; destination restored when the returned value is deleted.
+  common::Restorer<Messages *> DiscardMessages() {
+    return common::ScopedSet(messages_, nullptr);
+  }
 
   template<typename... A> Message *Say(CharBlock at, A &&... args) {
     if (messages_ != nullptr) {