Serialize library level into .swiftmodule binary format

Add a LIBRARY_LEVEL record to the .swiftmodule options block so the
declared -library-level value survives across compilations. Without
this, imported modules have to always fell back to a path heuristic
that could only distinguish API vs SPI and never returned IPI.

rdar://174255626

lib/Serialization/SerializedModuleLoader.cpp

@@ -997,6 +997,7 @@ LoadedFile *SerializedModuleLoaderBase::loadAST(
       M.setDeferredCodeGen();
     if (loadedModuleFile->isAggressiveCMOEnabled())
       M.setAggressiveCMOEnabled();
+    M.setStoredLibraryLevel(loadedModuleFile->getLibraryLevel());
     if (loadedModuleFile->hasCxxInteroperability()) {
       M.setHasCxxInteroperability();
       M.setCXXStdlibKind(loadedModuleFile->getCXXStdlibKind());