[AST] Lazily compute ProtocolDecl::getKnownProtocolKind()

Rather than eagerly doing a bunch of name lookups to establish the known
protocol kind, lazily match the ProtocolDecl to the list of known
protocols as-needed. This eliminates a bunch of up-front unqualified
name lookups when spinning up a type checker.

lib/Frontend/Frontend.cpp

@@ -803,11 +803,6 @@ void CompilerInstance::parseAndCheckTypesUpTo(
                         options.SwitchCheckingInvocationThreshold);
   });
 
-  // Even if there were no source files, we should still record known
-  // protocols.
-  if (auto *stdlib = Context->getStdlibModule())
-    Context->recordKnownProtocols(stdlib);
-
   if (Invocation.isCodeCompletion()) {
     performDelayedParsing(MainModule, PersistentState,
                           Invocation.getCodeCompletionFactory());