CodeGen: correct block mangling in ObjC

Mangling for blocks defined within blocks in an ObjectiveC context were also
broken by SVN r219393.  Because the code in mangleName assumed that the code was
either C or C++, we would trigger assertions when trying to mangle the inner
blocks in an ObjectiveC context.

Add a test and use the ObjectiveC specific mangling when dealing with an
ObjectiveC method declaration.

llvm-svn: 219697