Frontend: Deprecate -warn-on-potentially-unavailable-enum-case option.

When `-warn-on-potentially-unavailable-enum-case` was introduced, the build
system was required to invoke `swift-frontend` at artificially low deployment
targets when emitting `.swiftinterface` files for legacy architectures. Because
the deployment target was low, some availability diagnostics needed to be
de-fanged in order to allow module interface emission to succeed. Today, the
build system is able to use the correct deployment target when emitting module
interfaces and the `-warn-on-potentially-unavailable-enum-case` is superfluous,
so deprecate it.

Resolves rdar://114092047

lib/Frontend/CompilerInvocation.cpp

@@ -642,6 +642,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   if (Args.hasArg(OPT_check_api_availability_only))
     Diags.diagnose(SourceLoc(), diag::warn_flag_deprecated,
                    "-check-api-availability-only");
+  if (Args.hasArg(OPT_warn_on_potentially_unavailable_enum_case))
+    Diags.diagnose(SourceLoc(), diag::warn_flag_deprecated,
+                   "-warn-on-potentially-unavailable-enum-case");
 
   if (const Arg *A = Args.getLastArg(OPT_unavailable_decl_optimization_EQ)) {
     auto value =
@@ -667,8 +670,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
       = A->getOption().matches(OPT_enable_conformance_availability_errors);
   }
 
-  Opts.WarnOnPotentiallyUnavailableEnumCase |=
-      Args.hasArg(OPT_warn_on_potentially_unavailable_enum_case);
   Opts.WarnOnEditorPlaceholder |= Args.hasArg(OPT_warn_on_editor_placeholder);
 
   if (auto A = Args.getLastArg(OPT_disable_typo_correction,