NFC: simplify test and add assertion

This is a follow-up to clean-up swiftlang#74372

Author: kavon

lib/AST/Decl.cpp

@@ -1827,6 +1827,7 @@ bool ExtensionDecl::isWrittenWithConstraints() const {
   // If the type has no inverse requirements, there are no extra constraints
   // to write.
   if (typeInverseReqs.empty()) {
+    assert(extInverseReqs.empty() && "extension retroactively added inverse?");
     return false;
   }
 

validation-test/execution/issue-72719.swift renamed to validation-test/IRGen/issue-72719.swift

@@ -1,7 +1,5 @@
-// RUN: %target-swift-frontend -interpret %s
-// REQUIRES: executable_test
+// RUN: %target-swift-frontend -emit-ir -g %s > /dev/null
 
-// This only reproduced if you provide the -interpret flag!
 // https://github.com/apple/swift/issues/72719
 
 protocol D {}