[InterfaceGen] Print property initializers in resilient, fixed-layout types (#19619)

Augment the ASTPrinter to print the name and text of initializer expressions if
a property has an initializer and the type is @_fixed_layout and resides in a resilient module, and serialize the text for partial modules.

With this change, all .swiftinterface files in the project (except for SwiftLang) compile to swiftmodules on macOS.

rdar://43774580
rdar://43812188

include/swift/Serialization/ModuleFile.h

@@ -887,6 +887,9 @@ public:
 
   /// Reads inlinable body text from \c DeclTypeCursor, if present.
   Optional<StringRef> maybeReadInlinableBodyText();
+
+  /// Reads pattern initializer text from \c DeclTypeCursor, if present.
+  Optional<StringRef> maybeReadPatternInitializerText();
 };
 
 template <typename T, typename RawData>