Eliminate Sema::CheckValueInitialization; its callers now use

InitializationSequence to perform the actual initialization.

Also, introduced the notion of a tree of initialized entities, so that
we can know where an initialization began when dealing with nested
initializations (as occur when performing list initialization). This
will, eventually, be useful for producing better diagnostics when list
initialization fails, because we can show the path from the top-level
object being initialized down to the actual subobject where
initialization failed.

llvm-svn: 91516