Merge pull request #61184 from xymus/default-require-explicit-avail-reland

[Sema] Require explicit availability on public modules (Landing again after we held it back)

lib/Frontend/CompilerInvocation.cpp

@@ -735,7 +735,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
                      diagLevel);
     }
   } else if (Args.getLastArg(OPT_require_explicit_availability,
-                             OPT_require_explicit_availability_target)) {
+                             OPT_require_explicit_availability_target) ||
+             Opts.LibraryLevel == LibraryLevel::API) {
     Opts.RequireExplicitAvailability = DiagnosticBehavior::Warning;
   }