Create proper Objective-C @encoding for C++ classes; fixes rdar://9357400.

Go through and expand the members of bases into the encoding string (and encode the VTable as well).
Unlike gcc which expands virtual bases as many times as they appear in the
hierarchy, clang will only expand them once at the end, to reflect the actual layout.

Note that there doesn't seem to be a way to indicate in the encoding that
packing/alignment of members is different that normal, in which case
the encoding will be out-of-sync with the real layout.
If the runtime switches to just consider the size of types without
taking into account alignment, we could easily make padding explicit in the
encoding (e.g. using arrays of chars). The encoding strings would be
longer then though.

Also encode a flexible array member as array of 0 size, like gcc, not as a pointer.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@131365 91177308-0d34-0410-b5e6-96231b3b80d8