[llvm] [BOLT][DWARF][NFC] Split processUnitDIE into two lambdas (PR #99225)

[Amir Ayupov via llvm-commits]

aaupov wrote:

Heads up that this diff is not NFC: https://lab.llvm.org/buildbot/#/builders/92/builds/2042

https://github.com/llvm/llvm-project/pull/99225