Objective C / C++ Union Support

[bosborn]

Any chance of supporting union similar to struct in ObjCDeclarationKind.swift?

Running jazzy version 0.14.0 with a C++ dependency already built, receiving the following error:
SourceKittenFramework/ObjCDeclarationKind.swift:81: Fatal error: Unsupported CXCursorKind: UnionDecl

Manually changing union in the offending file to struct allows documentation to generate.

Example:
typedef union { double a; int b; char *c; } VALUE;
to
typedef struct { double a; int b; char *c; } VALUE;

[johnfairh]

There's a good chance all this needs is adding CXCursor_UnionDecl in that file and a quick testcase in ClangTranslationUnitTests -- this does look to be a missing ObjC feature though can't be that common in practice as I don't remember hearing a request for it before.

[bosborn]

Took a shot at it in #706. Not too familiar with the testing framework, so based it on the results from using struct.