Ensure that variant part discriminator is read by MetadataLoader

https://reviews.llvm.org/D42082 introduced variant parts to debug info
in LLVM. Subsequent work on the Rust compiler has found a bug in that
patch; namely, there is a path in MetadataLoader that fails to restore
the discriminator.

This patch fixes the bug.

Patch by: Tom Tromey

Differential revision: https://reviews.llvm.org/D52340

llvm-svn: 342725

llvm/lib/Bitcode/Reader/MetadataLoader.cpp

@@ -1313,7 +1313,7 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
                            (Context, Tag, Name, File, Line, Scope, BaseType,
                             SizeInBits, AlignInBits, OffsetInBits, Flags,
                             Elements, RuntimeLang, VTableHolder, TemplateParams,
-                            Identifier));
+                            Identifier, Discriminator));
     if (!IsNotUsedInTypeRef && Identifier)
       MetadataList.addTypeRef(*Identifier, *cast<DICompositeType>(CT));