[TypeChecker] Mark type repr as invalid if IUO appears in incorrect position

Mark IUO repr in incorrect position as invalid. This is the only
way to indicate that something went wrong without supressing
checking other reprs in the same type. For example:

```swift
struct S<T, U> { ... }

_ = S<Int!, String!>(...)
```

Compiler should diagnose both `Int!` and `String!` as invalid,
but returning `ErrorType` from here would stop type resolution
after `Int!`.

Author: xedin

lib/Sema/TypeCheckType.cpp

@@ -3790,6 +3790,19 @@ NeverNullType TypeResolver::resolveImplicitlyUnwrappedOptionalType(
   if (doDiag && !options.contains(TypeResolutionFlags::SilenceErrors)) {
     // Prior to Swift 5, we allow 'as T!' and turn it into a disjunction.
     if (getASTContext().isSwiftVersionAtLeast(5)) {
+      // Mark this repr as invalid. This is the only way to indicate that
+      // something went wrong without supressing checking other reprs in
+      // the same type. For example:
+      //
+      // struct S<T, U> { ... }
+      //
+      // _ = S<Int!, String!>(...)
+      //
+      // Compiler should diagnose both `Int!` and `String!` as invalid,
+      // but returning `ErrorType` from here would stop type resolution
+      // after `Int!`.
+      repr->setInvalid();
+
       diagnose(repr->getStartLoc(),
                diag::implicitly_unwrapped_optional_in_illegal_position)
           .fixItReplace(repr->getExclamationLoc(), "?");

test/Sema/diag_erroneous_iuo.swift

@@ -61,11 +61,8 @@ func genericFunctionSigil<T>(
 }
 
 func genericFunctionSigilArray<T>(
-  // FIXME: We validate these types multiple times resulting in multiple diagnostics
   iuo: [T!] // expected-error {{'!' is not allowed here; perhaps '?' was intended?}}{{10-11=?}}
-  // expected-error@-1 {{'!' is not allowed here; perhaps '?' was intended?}}{{10-11=?}}
 ) -> [T!] { // expected-error {{'!' is not allowed here; perhaps '?' was intended?}}{{8-9=?}}
-  // expected-error@-1 {{'!' is not allowed here; perhaps '?' was intended?}}{{8-9=?}}
   return iuo
 }