Revert "[demangler] Simplify OutputBuffer initialization"

Reverting due to a bot failure:
https://lab.llvm.org/buildbot/#/builders/5/builds/22738

This reverts commit 5b3ca24a35e91bf9c19af856e7f92c69b17f989e.

GitOrigin-RevId: aabeb5eb7f0aaa2c80147d904959c882cdeba1e5
2 files changed