WGSL: Run SeparateCompoundStructDeclarations to name structs Borrow SeparateCompoundStructDeclarations from the Metal backend to separate struct definitions and name nameless structs. Refactor id generation into a callback. Bug: angleproject:42267100 Change-Id: I8ed6f13113a5d5e450d30ce187ce1df52e572000 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5889089 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Matthew Denton <mpdenton@chromium.org>