Fix an assertion under -disable-availability-checking in zippered mode

Author: rjmccall

lib/SILGen/SILGenDecl.cpp

@@ -1896,7 +1896,8 @@ void SILGenFunction::emitStmtCondition(StmtCondition Cond, JumpDest FalseDest,
       assert(!OSVersion.isEmpty()
              || getASTContext().LangOpts.DisableAvailabilityChecking);
 
-      if (getASTContext().LangOpts.TargetVariant) {
+      if (getASTContext().LangOpts.TargetVariant &&
+          !getASTContext().LangOpts.DisableAvailabilityChecking) {
         // We're building zippered, so we need to pass both macOS and iOS
         // versions to the the runtime version range check. At run time
         // that check will determine what kind of process this code is loaded

test/SILGen/availability_disabled_zippered.swift

@@ -0,0 +1,6 @@
+// RUN: %target-swift-emit-silgen -target %target-cpu-apple-macosx13 -target-variant %target-cpu-apple-ios16-macabi -disable-availability-checking %s -verify
+// REQUIRES: OS=macosx
+
+func foo() {
+    if #available(macOS 10.15, *) {}
+}