Replace HashStringUsingDJB with llvm::djbHash

Summary:
The llvm function is equivalent to this one. Where possible I tried to
replace const char* with llvm::StringRef to avoid extra strlen
computations. In most places, I was able to track the c string back to
the ConstString it was created from.

I also create a test that verifies we are able to lookup names with
unicode characters, as a bug in the llvm compiler (it accidentally used
a different hash function) meant this was not working until recently.

This also removes the unused ExportTable class.

Reviewers: aprantl, davide

Subscribers: JDevlieghere, lldb-commits

Differential Revision: https://reviews.llvm.org/D43596

llvm-svn: 325927