Fixed two bugs in CFG construction:

BUG 1) 

CFG failed to build for empty functions, or functions containing only
NullStmts or empty compound statements.

We now handle such cases, although now we cannot test for CFG
construction failure by asserting that the last block constructed is
not NULL (since it now may be).

BUG 2)

CFG construction segfaulted on some cases when walking the AST and not
taking into account that some children of a statement may be NULL.

llvm-svn: 42370