[DeadArgumentElim] Clone function level metadatas

Some Function level metadatas, such as function entry count, are not cloned in
DeadArgumentElim. This happens a lot in lto/thinlto because of DeadArgumentElim
after internalization.

This patch clones the metadatas in the original function to the new function.

Differential Revision: https://reviews.llvm.org/D44127

llvm-svn: 328991