Rollup merge of #39528 - dylanmckay:llvm-4.0-diglobalvar, r=alexcrichton

[LLVM 4.0] Support a debug info API change for LLVM 4.0

Instead of directly creating a `DIGlobalVariable`, we now have to create
a `DIGlobalVariableExpression` which itself contains a reference to a
'DIGlobalVariable'.

This is a straightforward change.

In the future, we should rename `DIGlobalVariable` in the FFI
bindings, assuming we will only refer to `DIGlobalVariableExpression`
and not `DIGlobalVariable`.