[SE-0497] Implement @export attribute syntax

Implement the @export(implementation) and @export(interface) attributes
to replace @_alwaysEmitIntoClient and @_neverEmitIntoClient. Provide a
warning + Fix-It to start staging out the very-new
@_neverEmitIntoClient. We'll hold off on pushing folks toward
@_alwaysEmitIntoClient for a little longer.

lib/Serialization/ModuleFormat.h

@@ -58,7 +58,7 @@ const uint16_t SWIFTMODULE_VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t SWIFTMODULE_VERSION_MINOR = 972; // SIL asmname string table
+const uint16_t SWIFTMODULE_VERSION_MINOR = 973; // @export attribute
 
 /// A standard hash seed used for all string hashes in a serialized module.
 ///
@@ -2412,6 +2412,11 @@ namespace decls_block {
     BCFixed<2>  // inline value
   >;
 
+  using ExportDeclAttrLayout = BCRecordLayout<
+    Export_DECL_ATTR,
+    BCFixed<1>  // export kind value
+  >;
+
   using NonSendableDeclAttrLayout = BCRecordLayout<
     NonSendable_DECL_ATTR,
     BCFixed<1>  // non-sendable kind