[Frontend] Enforce restrictions on private interfaces

Previously we may have silently bailed if a private
module interface path was specified, but the
frontend action didn't support it. Instead, make
sure we enforce the same restrictions that we
enforce for public module interfaces.

Author: hamishknight

lib/Frontend/ArgsToFrontendOptionsConverter.cpp

@@ -539,7 +539,8 @@ bool ArgsToFrontendOptionsConverter::checkUnusedSupplementaryOutputPaths()
      return true;
    }
   if (!FrontendOptions::canActionEmitInterface(Opts.RequestedAction) &&
-      Opts.InputsAndOutputs.hasModuleInterfaceOutputPath()) {
+      (Opts.InputsAndOutputs.hasModuleInterfaceOutputPath() ||
+       Opts.InputsAndOutputs.hasPrivateModuleInterfaceOutputPath())) {
     Diags.diagnose(SourceLoc(), diag::error_mode_cannot_emit_interface);
     return true;
   }

test/Frontend/supplementary-output-support.swift

@@ -28,3 +28,8 @@
 // PARSE_NO_INTERFACE: error: this mode does not support emitting module interface files{{$}}
 // RUN: not %target-swift-frontend -emit-silgen -emit-module-interface-path %t %s 2>&1 | %FileCheck -check-prefix=SILGEN_NO_INTERFACE %s
 // SILGEN_NO_INTERFACE: error: this mode does not support emitting module interface files{{$}}
+
+// RUN: not %target-swift-frontend -parse -emit-private-module-interface-path %t %s 2>&1 | %FileCheck -check-prefix=PARSE_NO_PRIVATE_INTERFACE %s
+// PARSE_NO_PRIVATE_INTERFACE: error: this mode does not support emitting module interface files{{$}}
+// RUN: not %target-swift-frontend -emit-silgen -emit-private-module-interface-path %t %s 2>&1 | %FileCheck -check-prefix=SILGEN_NO_PRIVATE_INTERFACE %s
+// SILGEN_NO_PRIVATE_INTERFACE: error: this mode does not support emitting module interface files{{$}}