Rewrite default initialization of anonymous structs/unions within a

constructor. Previously, we did some bogus recursion into the fields
of anonymous structs (recursively), which ended up building invalid
ASTs that would cause CodeGen to crash due to invalid GEPs.

Now, we instead build the default initializations based on the
indirect field declarations at the top level, which properly generates
the sequence of GEPs needed to initialize the proper member. Fixes
PR10512 and <rdar://problem/9924046>.

llvm-svn: 137212