[Serialization] Allow rebuilding modules from the resource dir on SDK mismatch

Modules loaded from the resource dir are not usually rebuilt from the
swiftinterface as it would indicate a configuration problem. Lift that
behavior for SDK mismatch and still rebuild them.

This use case applies when a toolchain is used with a different SDK than
the one use to build the modules in the toolchain.

rdar://106101760

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -790,7 +790,9 @@ class ModuleInterfaceLoaderImpl {
           UsableModulePath = adjacentMod;
           return std::make_error_code(std::errc::not_supported);
         } else if (isInResourceDir(adjacentMod) &&
-                   loadMode == ModuleLoadingMode::PreferSerialized) {
+                   loadMode == ModuleLoadingMode::PreferSerialized &&
+                   rebuildInfo.getOrInsertCandidateModule(adjacentMod).serializationStatus !=
+                     serialization::Status::SDKMismatch) {
           // Special-case here: If we're loading a .swiftmodule from the resource
           // dir adjacent to the compiler, defer to the serialized loader instead
           // of falling back. This is mainly to support development of Swift,
@@ -798,6 +800,8 @@ class ModuleInterfaceLoaderImpl {
           // recompile the standard library. If that happens, don't fall back
           // and silently recompile the standard library -- instead, error like
           // we used to.
+          // Still accept modules built with a different SDK, allowing the use
+          // of one toolchain against a different SDK.
           LLVM_DEBUG(llvm::dbgs() << "Found out-of-date module in the "
                                      "resource-dir at "
                                   << adjacentMod