[InterfaceGen] Print bodies of inlinable functions in textual interfaces (#19224)

* Introduce stored inlinable function bodies

* Remove serialization changes

* [InterfaceGen] Print inlinable function bodies

* Clean up a little bit and add test

* Undo changes to InlinableText

* Add serialization and deserialization for inlinable body text

* Allow parser to parse accessor bodies in interfaces

* Fix some tests

* Fix remaining tests

* Add tests for usableFromInline decls

* Add comments

* Clean up function body printing throughout

* Add tests for subscripts

* Remove comment about subscript inlinable text

* Address some comments

* Handle lack of @objc on Linux

include/swift/Serialization/ModuleFile.h

@@ -884,6 +884,9 @@ public:
 
   /// Reads a foreign error conformance from \c DeclTypeCursor, if present.
   Optional<ForeignErrorConvention> maybeReadForeignErrorConvention();
+
+  /// Reads inlinable body text from \c DeclTypeCursor, if present.
+  Optional<StringRef> maybeReadInlinableBodyText();
 };
 
 template <typename T, typename RawData>