[llvm] [BOLT] Create and release .dwo DWARF contexts incrementally (PR #214900)

[Alexandros Lamprineas via llvm-commits]

labrinea wrote:

@rafaelauler both the [nfc](https://lab.llvm.org/buildbot/#/builders/217/builds/19007) and [dylib](https://lab.llvm.org/buildbot/#/builders/216/builds/6127) builders have been experiencing intermittent failures of `BOLT::dwarf5-ftypes-dwp-input-dwo-output.test` and I am suspecting this change may relate to them. Looking at `DWARFRewriter::processBucket` if DWOId exists but SplitCU is std::nullopt, BOLT silently skips ProcessSplitCU(), so no main.dwo.dwo gets emitted? I am not sure..

https://github.com/llvm/llvm-project/pull/214900