Revert "Allow existential opening for parameters of optional type."

This reverts commit 6e7fff7e65283ae9b25116fa6b75ba92fd2f2a58. The
asymmetry between opening for optional parameters and optional
arguments is surprising enough that we're currently opting not to
allow them.

Author: DougGregor

lib/Sema/CSSimplify.cpp

@@ -1402,10 +1402,8 @@ shouldOpenExistentialCallArgument(
   if (param->isVariadic() && !param->getVarargBaseTy()->hasTypeSequence())
     return None;
 
-  // Look through an inout and optional types on the formal type of the
-  // parameter.
-  auto formalParamTy = param->getInterfaceType()->getInOutObjectType()
-      ->lookThroughSingleOptionalType();
+  // Look through an inout type on the formal type of the parameter.
+  auto formalParamTy = param->getInterfaceType()->getInOutObjectType();
 
   // If the argument is of an existential metatype, look through the
   // metatype on the parameter.
@@ -1414,8 +1412,8 @@ shouldOpenExistentialCallArgument(
     paramTy = paramTy->getMetatypeInstanceType();
   }
 
-  // Look through an inout and optional types on the parameter.
-  paramTy = paramTy->getInOutObjectType()->lookThroughSingleOptionalType();
+  // Look through an inout type on the parameter.
+  paramTy = paramTy->getInOutObjectType();
 
   // The parameter type must be a type variable.
   auto paramTypeVar = paramTy->getAs<TypeVariableType>();

test/Constraints/opened_existentials.swift

@@ -159,16 +159,6 @@ func passesInOut(i: Int) {
   takesInOut(&p)
 }
 
-func takesOptional<T: P>(_ value: T?) { }
-// expected-note@-1{{required by global function 'takesOptional' where 'T' = 'P'}}
-
-func passesToOptional(p: any P, pOpt: (any P)?) {
-  takesOptional(p) // okay
-  takesOptional(pOpt) // expected-error{{protocol 'P' as a type cannot conform to the protocol itself}}
-  // expected-note@-1{{only concrete types such as structs, enums and classes can conform to protocols}}
-}
-
-
 @available(SwiftStdlib 5.1, *)
 func testReturningOpaqueTypes(p: any P) {
   let q = p.getQ()