Allow -swift-version 6 in asserts compilers only

To allow us to start testing language changes tied to a future Swift 6 mode without actually *shipping* a Swift 6 mode to customers who might accidentally use it before it's ready.

This commit also adds parallel tests for a number of already existing (but untested) Swift 6 mode behaviors.

Author: beccadax

lib/Basic/Version.cpp

@@ -326,6 +326,17 @@ Optional<Version> Version::getEffectiveLanguageVersion() const {
     static_assert(SWIFT_VERSION_MAJOR == 5,
                   "getCurrentLanguageVersion is no longer correct here");
     return Version::getCurrentLanguageVersion();
+  case 6:
+    // Allow version '6' in asserts compilers *only* so that we can start
+    // testing changes slated for Swift 6. Note that it's still not listed in
+    // `Version::getValidEffectiveVersions()`.
+    // FIXME: When Swift 6 becomes real, remove 'REQUIRES: asserts' from tests
+    //        using '-swift-version 6'.
+#ifdef NDEBUG
+    LLVM_FALLTHROUGH;
+#else
+    return Version{6};
+#endif
   default:
     return None;
   }