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.

lib/Serialization/SerializedModuleLoader.cpp

@@ -492,6 +492,11 @@ OperatorDecl *SerializedASTFile::lookupOperator(Identifier name,
   return File.lookupOperator(name, fixity);
 }
 
+PrecedenceGroupDecl *
+SerializedASTFile::lookupPrecedenceGroup(Identifier name) const {
+  return File.lookupPrecedenceGroup(name);
+}
+
 void SerializedASTFile::lookupVisibleDecls(Module::AccessPathTy accessPath,
                                            VisibleDeclConsumer &consumer,
                                            NLKind lookupKind) const {