Implement SE-0077: precedence group declarations.

What I've implemented here deviates from the current proposal text
in the following ways:

- I had to introduce a FunctionArrowPrecedence to capture the parsing
  of -> in expression contexts.

- I found it convenient to continue to model the assignment property
  explicitly.

- The comparison and casting operators have historically been
  non-associative; I have chosen to preserve that, since I don't
  think this proposal intended to change it.

- This uses the precedence group names and higherThan/lowerThan
  as agreed in discussion.

include/swift/Serialization/SerializedModuleLoader.h

@@ -123,6 +123,9 @@ public:
   virtual OperatorDecl *lookupOperator(Identifier name,
                                        DeclKind fixity) const override;
 
+  virtual PrecedenceGroupDecl *
+  lookupPrecedenceGroup(Identifier name) const override;
+
   virtual void lookupVisibleDecls(Module::AccessPathTy accessPath,
                                   VisibleDeclConsumer &consumer,
                                   NLKind lookupKind) const override;