[CodeComplete] Allow getDeclaration on RK_Pattern result.

[ioeric]

Summary

RK_Pattern results can also have associated declarations e.g. field
decls in constructor initializers.

[bkramer]

lg

include/clang/Sema/CodeCompleteConsumer.h

CodeCompletionResult(CodeCompletionString *Pattern, const NamedDecl *D,
                        unsigned Priority)
       : Declaration(D), Pattern(Pattern), Priority(Priority), Kind(RK_Pattern),
         Hidden(false), QualifierIsInformative(false),
         StartsNestedNameSpecifier(false), AllParametersAreInformative(false),
         DeclaringEntity(false) {
     computeCursorKindAndAvailability();
   }
 
-  /// Retrieve the declaration stored in this result.
+  /// Retrieve the declaration stored in this result. This might be nullptr if
+  /// Kind is RK_Pattern.
   const NamedDecl *getDeclaration() const {
-    assert(Kind == RK_Declaration && "Not a declaration result");
+    assert(((Kind == RK_Declaration) || (Kind == RK_Pattern)) &&
+           "Not a declaration or pattern result");
     return Declaration;
   }
 
   /// Retrieve the keyword stored in this result.
   const char *getKeyword() const {
     assert(Kind == RK_Keyword && "Not a keyword result");
     return Keyword;
   }