[Diagnostics] Update existential type error message for clarity

Rework solution

to be squashed

Author: mazevedofs

include/swift/AST/DiagnosticsSema.def

@@ -4793,7 +4793,7 @@ ERROR(unchecked_not_existential,none,
       "'unchecked' attribute cannot apply to non-protocol type %0", (Type))
 
 ERROR(redundant_any_in_existential,none,
-       "redundant 'any' has no effect on existential type %0",
+       "redundant 'any' in type %0",
        (Type))
 ERROR(any_not_existential,none,
        "'any' has no effect on %select{concrete type|type parameter}0 %1",

lib/AST/ASTContext.cpp

@@ -4316,8 +4316,6 @@ Type ExistentialType::get(Type constraint) {
   if (constraint->is<ExistentialMetatypeType>())
     return constraint;
 
-  assert(constraint->isConstraintType());
-
   bool printWithAny = true;
   if (constraint->isEqual(C.TheAnyType) || constraint->isAnyObject())
     printWithAny = false;

lib/Sema/TypeCheckType.cpp

@@ -4237,7 +4237,8 @@ TypeResolver::resolveExistentialType(ExistentialTypeRepr *repr,
     // Diagnose redundant `any` on an already existential type e.g. any (any P)
     // with a fix-it to remove first any.
     if (constraintType->is<ExistentialType>()) {
-      diagnose(repr->getLoc(), diag::redundant_any_in_existential, constraintType)
+      diagnose(repr->getLoc(), diag::redundant_any_in_existential,
+               ExistentialType::get(constraintType))
           .fixItRemove(repr->getAnyLoc());
       return constraintType;
     }

test/type/explicit_existential.swift

@@ -334,12 +334,12 @@ func testEnumAssociatedValue() {
 
 // https://github.com/apple/swift/issues/58920
 typealias Iterator = any IteratorProtocol
-var example: any Iterator = 5 // expected-error{{redundant 'any' has no effect on existential type 'Iterator' (aka 'any IteratorProtocol')}} {{14-18=}} 
+var example: any Iterator = 5 // expected-error{{redundant 'any' in type 'any Iterator' (aka 'any any IteratorProtocol')}} {{14-18=}}
 // expected-error@-1{{value of type 'Int' does not conform to specified type 'IteratorProtocol'}}
-var example1: any (any IteratorProtocol) = 5 // expected-error{{redundant 'any' has no effect on existential type 'any IteratorProtocol'}} {{15-19=}}
+var example1: any (any IteratorProtocol) = 5 // expected-error{{redundant 'any' in type 'any (any IteratorProtocol)'}} {{15-19=}}
 // expected-error@-1{{value of type 'Int' does not conform to specified type 'IteratorProtocol'}}
 
 protocol PP {}
 struct A : PP {}
 let _: any PP = A() // Ok
-let _: any (any PP) = A() // expected-error{{redundant 'any' has no effect on existential type 'any PP'}} {{8-12=}}
+let _: any (any PP) = A() // expected-error{{redundant 'any' in type 'any (any PP)'}} {{8-12=}}