IR: Extract macros from DILocation, NFC
`DILocation` is a lightweight wrapper. Its accessors check for null and
the correct type, and then forward to `MDLocation`.
Extract a couple of macros to do the `dyn_cast_or_null<>` and default
return logic. I'll be using these to minimize error-prone boilerplate
when I move the new hierarchy into place -- since all the other
subclasses of `DIDescriptor` will similarly become lightweight wrappers.
(Note that I hope to obsolete these wrappers fairly quickly, with the
goal of renaming the underlying types (e.g., I'll rename `MDLocation` to
`DILocation` once the name is free).)
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229953
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