[ADT] Fix an empty BitVector call getData assert `idx < size()' failed

[wingrez]

Fixes #65500

[zero9178]

Looks good!

Could you add a test exercising this code in https://github.com/llvm/llvm-project/blob/622354a522073b0a048a88c957b161fb376a40eb/llvm/unittests/ADT/BitVectorTest.cpp as well?
While you cannot do a lot with the result, checking that no assert is trigerred probably makes sense.
So I think just calling .data() and putting a comment that says something like "Make sure calling .data() is legal even on an empty BitVector" to make sure that we do not regress would be great.

[kuhar]

+1 to @zero9178's request

So I think just calling .data() and putting a comment that says something like "Make sure calling .data() is legal even on an empty BitVector" to make sure that we do not regress would be great.

We can also assert that that the arrayref is .empty().

[kuhar]

LGTM