[dsymutil] Upstream unobfuscation logic.

The unobufscation support for BCSymbolMaps was the last piece of code
that hasn't been upstreamed yet. This patch contains a reworked version
of the existing code and relevant tests.

Differential revision: https://reviews.llvm.org/D56346

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350580 91177308-0d34-0410-b5e6-96231b3b80d8
21 files changed