Fix and enable generating general union initializers. Essentially, what

this does is reconstruct the type for structs and arrays if the type 
wouldn't be compatible otherwise.

The assertion about packing in the struct type reconstruction code 
sucks, but I don't see any obvious way to fix it.  Maybe we need a general
utility method to take a list of types and alignments and try to construct an
unpacked type if possible?

llvm-svn: 51785