[Serialization] Drop overriding properties with missing bases.

Like the previous commit, but with added trickiness because we also
serialize the form of the PatternBindingDecl a property came from.
Make getPattern handle a failure in the simple case that overrides
use, and pass that up to the PatternBindingDecl initialization. (This
can result in zero-element PatternBindingDecls, but that's fine.)

'getPattern' is also a change from 'maybeGetPattern', but every caller
knows how many patterns it expects, so accomodating the "maybe" case
is no longer important.

include/swift/Serialization/ModuleFile.h

@@ -503,9 +503,7 @@ private:
   bool readCommentBlock(llvm::BitstreamCursor &cursor);
 
   /// Recursively reads a pattern from \c DeclTypeCursor.
-  ///
-  /// If the record at the cursor is not a pattern, returns null.
-  Pattern *maybeReadPattern(DeclContext *owningDC);
+  llvm::Expected<Pattern *> readPattern(DeclContext *owningDC);
 
   ParameterList *readParameterList();