Fix classify_union to return Union for regular unions

Commit 623c7d7c4b accidentally changed the return value from
REGULAR_UNION to RegularEnum when converting string literals
to enum values. Commit b17670d3de then renamed RegularUnion
to Union, but the buggy return statement remained unchanged.
This caused unions to be misclassified as enums, preventing
LLDB from displaying union field contents.

src/etc/rust_types.py

@@ -130,4 +130,4 @@ def classify_union(fields: List) -> RustType:
         assert len(fields) == 1
         return RustType.CompressedEnum
     else:
-        return RustType.RegularEnum
+        return RustType.Union