[code-completion] Fix type context for single-expression implicit getter

This adds an implicit body so that we can dig out the return type
context the same way as a normal function. For now, we are also treating
the first expression in a multi-statement implicit getter body the same
way; we'll need to refactor how we complete in accessors to
differentiate those cases.

include/swift/Parse/CodeCompletionCallbacks.h

@@ -180,7 +180,7 @@ public:
       SmallVectorImpl<StringRef> &Keywords, SourceLoc introducerLoc) {};
 
   /// Complete at the beginning of accessor in a accessor block.
-  virtual void completeAccessorBeginning() {};
+  virtual void completeAccessorBeginning(CodeCompletionExpr *E) {};
 
   /// Complete the keyword in attribute, for instance, @available.
   virtual void completeDeclAttrKeyword(Decl *D, bool Sil, bool Param) {};