[Parser] Improve fix-it for subscription in protocol

https://bugs.swift.org/browse/SR-8340

Author: mzp

lib/Parse/ParseDecl.cpp

@@ -6226,7 +6226,7 @@ Parser::parseDeclSubscript(ParseDeclOptions Flags,
     if (!Status.isError()) {
       if (Flags.contains(PD_InProtocol)) {
         diagnose(Tok, diag::expected_lbrace_subscript_protocol)
-            .fixItInsertAfter(ElementTy.get()->getEndLoc(), " { get set }");
+            .fixItInsertAfter(ElementTy.get()->getEndLoc(), " { get <#set#> }");
       } else {
         diagnose(Tok, diag::expected_lbrace_subscript);
       }

test/decl/subscript/subscripting.swift

@@ -346,7 +346,7 @@ class Foo {
 protocol r23952125 {
   associatedtype ItemType
   var count: Int { get }
-  subscript(index: Int) -> ItemType  // expected-error {{subscript in protocol must have explicit { get } or { get set } specifier}} {{36-36= { get set \}}}
+  subscript(index: Int) -> ItemType  // expected-error {{subscript in protocol must have explicit { get } or { get set } specifier}} {{36-36= { get <#set#> \}}}
 
   var c : Int // expected-error {{property in protocol must have explicit { get } or { get set } specifier}}
 }