This macro applies always_inline in addition to inline. It also applies artificial, which lets debugger know that this is an artificial function. The used attribute is added in debug builds to ensure that the symbol is emitted in the binary so that LLDB can invoke it.
apple/swift#59072 accidentally changed the SWIFT_CLASS_NAMED macro to use `__attribute` when it previously used `__attribute__` (note the trailing underscores). While both keywords have the same semantics in clang, they are technically different tokens, so clang refuses to merge macro definitions that use one instead of the other; instead it would diagnose an ambiguity when a generated header from a new compiler imported a generated header from an old compiler. Change back to the old token to avoid this problem.
Fixes rdar://104252758.
