Mangling: treat C++ namespaces as enums

C++ namespaces are imported as Swift enums, so let's mangle them as enums.

This fixes lookup of nested namespaces, for example, `std.__1`: previously during demangling the `__1` decl was filtered out in `ASTBuilder::getAcceptableTypeDeclCandidate` as it expects `Demangle::Node::Kind::Structure` instead of `Demangle::Node::Kind::Enum`. This caused "Failed to reconstruct type for x" error.

Author: egorzhdan

lib/AST/ASTMangler.cpp

@@ -2303,9 +2303,9 @@ void ASTMangler::appendAnyGenericType(const GenericTypeDecl *decl) {
                isa<clang::ObjCCompatibleAliasDecl>(namedDecl)) {
       appendOperator("a");
     } else if (isa<clang::NamespaceDecl>(namedDecl)) {
-      // Note: Namespaces are not really structs, but since namespaces are
+      // Note: Namespaces are not really enums, but since namespaces are
       // imported as enums, be consistent.
-      appendOperator("V");
+      appendOperator("O");
     } else {
       llvm_unreachable("unknown imported Clang type");
     }

test/ClangImporter/cxx_interop_ir.swift

@@ -9,17 +9,17 @@ func indirectUsage() {
   useT(makeT())
 }
 
-// CHECK-LABEL: define hidden swiftcc %swift.type* @"$s6cxx_ir14reflectionInfo3argypXpSo2nsV10CXXInteropE1TV_tF"
-// CHECK: %0 = call swiftcc %swift.metadata_response @"$sSo2nsV10CXXInteropE1TVMa"({{i64|i32}} 0)
+// CHECK-LABEL: define hidden swiftcc %swift.type* @"$s6cxx_ir14reflectionInfo3argypXpSo2nsO10CXXInteropE1TV_tF"
+// CHECK: %0 = call swiftcc %swift.metadata_response @"$sSo2nsO10CXXInteropE1TVMa"({{i64|i32}} 0)
 func reflectionInfo(arg: namespacedT) -> Any.Type {
   return type(of: arg)
 }
 
-// CHECK: define hidden swiftcc void @"$s6cxx_ir24namespaceManglesIntoName3argySo2nsV10CXXInteropE1TV_tF"
+// CHECK: define hidden swiftcc void @"$s6cxx_ir24namespaceManglesIntoName3argySo2nsO10CXXInteropE1TV_tF"
 func namespaceManglesIntoName(arg: namespacedT) {
 }
 
-// CHECK: define hidden swiftcc void @"$s6cxx_ir42namespaceManglesIntoNameForUsingShadowDecl3argySo2nsV10CXXInteropE14NamespacedTypeV_tF"
+// CHECK: define hidden swiftcc void @"$s6cxx_ir42namespaceManglesIntoNameForUsingShadowDecl3argySo2nsO10CXXInteropE14NamespacedTypeV_tF"
 func namespaceManglesIntoNameForUsingShadowDecl(arg: NamespacedType) {
 }