This patch extends the previous patch by starting to incorporate more functionality...

This patch extends the previous patch by starting to incorporate more functionality, like lookup-by-name and exporting lookup tables, into the module manager.  Methods now have documentation.  A few more functions have been switched over to the new iterator style and away from manual/explicit iteration.  Ultimately we want to move away from name lookup here, as symlinks make filenames not a safe unique value, but we use it here as a stopgap before better measures are in place (namely instead using FileEntry* from a global FileManager).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@136107 91177308-0d34-0410-b5e6-96231b3b80d8