its vectors based on the subobject type we're initializing and the
(unstructured) initializer list. This eliminates some malloc thrashing
when parsing initializers (from 117 vector reallocations down to 0
when parsing Cocoa.h). We can't always pre-allocate the right amount
of storage, since designated initializers can cause us to initialize
in non-predictable patterns.

llvm-svn: 67421