Rollup merge of #155690 - fneddy:fix_classify_union, r=jieyouxu

Fix classify_union to return Union for regular unions

Commit 623c7d7c4bc accidentally changed the return value from REGULAR_UNION to RegularEnum when converting string literals to enum values. Commit b17670d3de2 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.

Author: tgross35