[embedded] Avoid compiler crash when using an opaque result type

[kubamracek]

We currently crash in IRGen when emitting metadata for an opaque type decl. We should not emit that in embedded Swift.

Assertion failed: (!isEmbedded(decl)), function forOpaqueTypeDescriptor, file Linking.h, line 928.
 #7 0x00000001083d8950 swift::irgen::IRGenModule::emitOpaqueTypeDecl(swift::OpaqueTypeDecl*) (.cold.3)
 #8 0x00000001035e5b80 swift::irgen::IRGenModule::emitOpaqueTypeDecl(swift::OpaqueTypeDecl*)
 #9 0x000000010362574c swift::irgen::IRGenModule::maybeEmitOpaqueTypeDecl(swift::OpaqueTypeDecl*)
#10 0x000000010355b0c0 swift::irgen::IRGenModule::emitSourceFile(swift::SourceFile&)
#11 0x0000000103660898 swift::IRGenRequest::evaluate(swift::Evaluator&, swift::IRGenDescriptor) const
#12 0x00000001036a69bc swift::GeneratedModule swift::SimpleRequest<swift::IRGenRequest, swift::GeneratedModule (swift::IRGenDescriptor), (swift::RequestFlags)9>::callDerived<0ul>(swift::Evaluator&, std::__1::integer_sequence<unsigned long, 0ul>) const
#13 0x0000000103668dbc llvm::Expected<swift::IRGenRequest::OutputType> swift::Evaluator::getResultUncached<swift::IRGenRequest>(swift::IRGenRequest const&)
#14 0x000000010366184c swift::performIRGeneration(swift::ModuleDecl*, swift::IRGenOptions const&, swift::TBDGenOptions const&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule>>, llvm::StringRef, swift::PrimarySpecificPaths const&, llvm::ArrayRef<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, llvm::GlobalVariable**)
#15 0x00000001031aefa8 generateIR(swift::IRGenOptions const&, swift::TBDGenOptions const&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule>>, swift::PrimarySpecificPaths const&, llvm::StringRef, llvm::PointerUnion<swift::ModuleDecl*, swift::SourceFile*>, llvm::GlobalVariable*&, llvm::ArrayRef<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>)
#16 0x00000001031ab6c4 performCompileStepsPostSILGen(swift::CompilerInstance&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule>>, llvm::PointerUnion<swift::ModuleDecl*, swift::SourceFile*>, swift::PrimarySpecificPaths const&, int&, swift::FrontendObserver*)
#17 0x00000001031aadd4 swift::performCompileStepsPostSema(swift::CompilerInstance&, int&, swift::FrontendObserver*)
#18 0x00000001031bb10c withSemanticAnalysis(swift::CompilerInstance&, swift::FrontendObserver*, llvm::function_ref<bool (swift::CompilerInstance&)>, bool)
#19 0x00000001031ad010 performCompile(swift::CompilerInstance&, int&, swift::FrontendObserver*)

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test