Annotating Reflection APIs Unavailable in Embedded

While I was trying to add Embedded support to SwiftSyntax, I ran into a bit of a rough edge in terms of Embedded support. I noticed that reflection APIs are conditionalized behind SWIFT_ENABLE_REFLECTION, which doesn't compose well with EmbeddedRestrictions diagnostics and @_unavailableInEmbedded. The Problem For one, enabling EmbeddedRestrictions diagnostics in non-Embedded mode doesn't catch uses of the reflection APIs, which makes Embedded support harder for libraries. // ✅ Fine with `Embed...