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/IDE/CodeCompletion.cpp

@@ -592,6 +592,8 @@ CodeCompletionResult::getCodeCompletionDeclKind(const Decl *D) {
     return CodeCompletionDeclKind::PrefixOperatorFunction;
   case DeclKind::PostfixOperator:
     return CodeCompletionDeclKind::PostfixOperatorFunction;
+  case DeclKind::PrecedenceGroup:
+    return CodeCompletionDeclKind::PrecedenceGroup;
   case DeclKind::EnumElement:
     return CodeCompletionDeclKind::EnumElement;
   case DeclKind::Subscript:
@@ -672,6 +674,9 @@ void CodeCompletionResult::print(raw_ostream &OS) const {
     case CodeCompletionDeclKind::Module:
       Prefix.append("[Module]");
       break;
+    case CodeCompletionDeclKind::PrecedenceGroup:
+      Prefix.append("[PrecedenceGroup]");
+      break;
     }
     break;
   case ResultKind::Keyword:
@@ -5398,6 +5403,7 @@ void swift::ide::copyCodeCompletionResults(CodeCompletionResultSink &targetSink,
       if (R->getKind() != CodeCompletionResult::Declaration)
         return false;
       switch(R->getAssociatedDeclKind()) {
+      case CodeCompletionDeclKind::PrecedenceGroup:
       case CodeCompletionDeclKind::Module:
       case CodeCompletionDeclKind::Class:
       case CodeCompletionDeclKind::Struct: