[Caching][NFC] Restructure CASOption in swift. NFC

Clean up how CASOptions are kept and passed inside swift to make the
code more readable. Also avoid a copy of CAS configuration in
ClangImporter.

unittests/FrontendTool/ModuleLoadingTests.cpp

@@ -103,8 +103,10 @@ protected:
     ClangImporterOptions clangImpOpts;
     symbolgraphgen::SymbolGraphOptions symbolGraphOpts;
     SILOptions silOpts;
+    CASOptions casOpts;
     auto ctx = ASTContext::get(langOpts, typecheckOpts, silOpts, searchPathOpts,
-                               clangImpOpts, symbolGraphOpts, sourceMgr, diags);
+                               clangImpOpts, symbolGraphOpts, casOpts,
+                               sourceMgr, diags);
 
     ctx->addModuleInterfaceChecker(
       std::make_unique<ModuleInterfaceCheckerImpl>(*ctx, cacheDir,