[Cloning] Clone every Function's Debug Info

Summary:
Export the CloneDebugInfoMetadata utility, which clones all debug info
associated with a function into the first module. Also use this function
in CloneModule on each function we clone (the CloneFunction entrypoint
already does this).

Without this, cloning a module will lead to DI quality regressions,
especially since r252219 reversed the Function <-> DISubprogram edge
(before we could get lucky and have this edge preserved if the
DISubprogram itself was, e.g. due to location metadata).

This was verified to fix missing debug information in julia and
a unittest to verify the new behavior is included.

Patch by Yichao Yu! Thanks!

Reviewers: loladiro, pcc
Differential Revision: http://reviews.llvm.org/D17165

llvm-svn: 260791