[Serialization] Soft-reject swiftmodules built against a different SDK

Change the way swiftmodules built against a different SDK than their
clients are rejected. This makes them silently ignored when the module
can be rebuilt from their swiftinterface, instead of reporting a hard
error.

rdar://93257769

lib/ASTSectionImporter/ASTSectionImporter.cpp

@@ -36,7 +36,7 @@ bool swift::parseASTSection(MemoryBufferSerializedModuleLoader &Loader,
   // headers. Iterate over all AST modules.
   while (!buf.empty()) {
     auto info = serialization::validateSerializedAST(
-        buf, Loader.isRequiredOSSAModules());
+        buf, Loader.isRequiredOSSAModules(), /*requiredSDK*/StringRef());
 
     assert(info.name.size() < (2 << 10) && "name failed sanity check");