Sema: Fix case where witness thrown error type is a subtype of a type parameter

In b300068, I changed the `if`
condition here to check for the absence of type variables as well
as type parameters. This is incorrect; the type variables come up
in ValueWitnessRequest, and the type parameters come up in
associated type inference. We want the matching to be more lax
in the former case.

Fixes rdar://149438520.

Author: slavapestov

lib/Sema/TypeCheckProtocol.cpp

@@ -912,8 +912,7 @@ RequirementMatch swift::matchWitness(
 
     case ThrownErrorSubtyping::Subtype:
       // If there were no type parameters, we're done.
-      if (!reqThrownError->hasTypeVariable() &&
-          !reqThrownError->hasTypeParameter())
+      if (!reqThrownError->hasTypeParameter())
         break;
 
       LLVM_FALLTHROUGH;

test/decl/protocol/conforms/typed_throws.swift

@@ -77,3 +77,17 @@ public protocol HasRethrowingMap: Sequence {
 }
 
 extension Array: HasRethrowingMap {}
+
+// rdar://149438520 -- incorrect handling of subtype relation between type parameter and Never
+protocol DependentThrowing {
+  associatedtype E: Error
+  func f() throws(E)
+}
+
+extension DependentThrowing {
+  func f() {}
+}
+
+struct DefaultDependentThrowing: DependentThrowing {
+  typealias E = Error
+}