Add workaround flag for lazy import-as-member

In rdar://123649082, a project failed to build because of the lazy import-as-member loading changes in #71320. That project was configured in a way that broke modularization and the correct solution is to fix it, but out of an abundance of caution, add a `-disable-named-lazy-import-as-member-loading` frontend flag in case a project needs to temporarily restore the old behavior.

As a bonus, this lets us write a test to verify that lazy import-as-member loading has positive performance impact.

include/swift/Basic/LangOptions.h

@@ -377,6 +377,11 @@ namespace swift {
     /// Enable experimental eager Clang module diagnostics.
     bool EnableExperimentalEagerClangModuleDiagnostics = false;
 
+    /// Force ClangImporter's import-as-member extensions to load thier members
+    /// immediately, bypassing their SwiftLookupTables. This emulates an
+    /// implementation quirk of previous compilers.
+    bool DisableNamedLazyImportAsMemberLoading = false;
+
     /// Enable inference of Sendable conformances for public types.
     bool EnableInferPublicSendable = false;