[clang] [llvm] [mlir] [MC][NFC] Make FeatureKV/SubtargetKV pointers private (PR #206178)

[Alexis Engelke via cfe-commits]

aengelke wrote:

[Entire planned change stack](https://github.com/aengelke/llvm-project/compare/49f787b8278d...featurekv) (5 commits).

In an all-target build, this change to FeatureKV will shrink .data.rel.ro by ~139 kiB, which currently need to be touched on every startup.

https://github.com/llvm/llvm-project/pull/206178