Add the start of DIE hashing for DWARF4 type units and split dwarf

Add the start of DIE hashing for DWARF4 type units and split dwarf
CUs.

Currently only hashes the name of CUs and the names of any children,
but it's an obvious first step to show the framework. The testcase
should continue to be correct, however, as it's an empty TU.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188243 91177308-0d34-0410-b5e6-96231b3b80d8