[llvm] In-class initialize DenseMapBase members. (PR #177168)

[Matt Davis via llvm-commits]

enferex wrote:

> Should we even care about this being initialized in the destructor?

I don't think the members are initialized in the destructor; rather, values from these members are being used in the dtor.  I think making the initialization in `DenseMapBase` clearer is helpful to engineers too, so there's the stylistic aspect of this change, as well as the side-effect of silencing the analyzer.

https://github.com/llvm/llvm-project/pull/177168