Sema: Simplify behavior of NameLookupFlags::PerformConformanceCheck flag

I'd like to remove this eventually, but for now I want
to remove this special case check.

lib/Sema/CSSimplify.cpp

@@ -3092,11 +3092,6 @@ performMemberLookup(ConstraintKind constraintKind, DeclName memberName,
     if (includeInaccessibleMembers)
       lookupOptions |= NameLookupFlags::IgnoreAccessibility;
 
-    // If a constructor is only visible as a witness for a protocol
-    // requirement, it must be an invalid override. Also, protocol
-    // extensions cannot yet define designated initializers.
-    lookupOptions -= NameLookupFlags::PerformConformanceCheck;
-
     LookupResult ctors = TC.lookupConstructors(DC, instanceTy, lookupOptions);
     if (!ctors)
       return result;    // No result.

lib/Sema/TypeCheckNameLookup.cpp

@@ -146,9 +146,7 @@ namespace {
       // If we found something within the protocol itself, and our
       // search began somewhere that is not in a protocol or extension
       // thereof, remap this declaration to the witness.
-      if (foundInType->is<ArchetypeType>() ||
-          foundInType->isExistentialType() ||
-          Options.contains(NameLookupFlags::PerformConformanceCheck)) {
+      if (Options.contains(NameLookupFlags::PerformConformanceCheck)) {
         // Dig out the protocol conformance.
         auto conformance = TC.conformsToProtocol(conformingType, foundProto, DC,
                                                  conformanceOptions);

test/Sema/diag_invalid_synthesized_init_proto_conformance.swift

@@ -8,14 +8,14 @@ class A : P { } // expected-error{{initializer requirement 'init()' can only be
 // No further errors
 
 class B : A {
-    init(x : Int) {} // expected-note {{'init(x:)' declared here}}
+    init(x : Int) {}
 }
 
 class C : B { }
 
 class D : B {
   init() {
-    super.init() // expected-error{{missing argument for parameter 'x' in call}}
+    super.init()
   }
 }