Add -emit-public-type-metadata-accessors to work around metadata linkage bug.

IRGen can introduce calls to type metadata accessors for types that
should not be visible to the current translate, which can manifest in
linker errors within a module (for references to private types when
whole module optimization is disabled) or across modules (for
references to private/internal types in another module). Introduce a
new compiler flag `-emit-public-type-metadata-accessors` that emits
all type metadata accessors with public linkage, to work around the
problem in affected projects. This flag is intended to go away once we
have a proper solution.

This bug has been around in Swift "forever", but compiling the
overlays using -enable-resilience has exacerbated the problem and
caused regressions. This is a short-term fix to
rdar://problem/40229755 while we work on the correct long-term fix.

lib/Frontend/CompilerInvocation.cpp

@@ -309,6 +309,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     }
   }
 
+  Opts.EmitPublicTypeMetadataAccessors =
+    Args.hasArg(OPT_emit_public_type_metadata_accessors);
+
   Opts.EnableNSKeyedArchiverDiagnostics =
       Args.hasFlag(OPT_enable_nskeyedarchiver_diagnostics,
                    OPT_disable_nskeyedarchiver_diagnostics,