Add a way to get the context of any particular scope.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187212 91177308-0d34-0410-b5e6-96231b3b80d8
Add preliminary support for hashing DIEs and breaking them into
type units.

Initially this support is used in the computation of an ODR checker
for C++. For now we're attaching it to the DIE, but in the future
it will be attached to the type unit.

This also starts breaking out types into the separation for type
units, but without actually splitting the DIEs.

In preparation for hashing the DIEs this adds a DIEString type
that contains a StringRef with the string contained at the label.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187213 91177308-0d34-0410-b5e6-96231b3b80d8