[ModuleInterface] Introduce env var to test the module-alias workaround

Set the env var SWIFT_ALIAS_MODULE_NAMES_IN_INTERFACES to turn on
generating swiftinterfaces with the AliasModuleNames workaround by
default.

Author: xymus

lib/Frontend/CompilerInvocation.cpp

@@ -371,7 +371,7 @@ static void ParseModuleInterfaceArgs(ModuleInterfaceOptions &Opts,
   Opts.AliasModuleNames |=
     Args.hasFlag(OPT_alias_module_names_in_module_interface,
                  OPT_disable_alias_module_names_in_module_interface,
-                 false);
+                 ::getenv("SWIFT_ALIAS_MODULE_NAMES_IN_INTERFACES"));
   Opts.PrintFullConvention |=
     Args.hasArg(OPT_experimental_print_full_convention);
   Opts.ExperimentalSPIImports |=