Alter the internal representation of the condition variable in

if/while/switch/for statements to ensure that walking the children of
these statements actually works. Previously, we stored the condition
variable as a VarDecl. However, StmtIterator isn't able to walk from a
VarDecl to a set of statements, and would (in some circumstances) walk
beyond the end of the list of statements, cause Bad Behavior.

In this change, we've gone back to representing the condition
variables as DeclStmts. While not as memory-efficient as VarDecls, it
greatly simplifies iteration over the children. 

Fixes the remainder of <rdar://problem/8104754>.

llvm-svn: 106504