Introduce a depth-first search of modules into the module manager,

which supports both pre-order and post-order traversal via a visitor
mechanism. Use this depth-first search with a post-order traversal to
give predictable ordering semantics when walking all of the lexical
declarations in the translation unit.

Eventually, module imports will occur in the source code rather than
at the beginning, and we'll have to revisit this walk.

llvm-svn: 138490