[llvm-cxxfilt] Improve strip-underscore behavior

Summary:
For platform that uses macho format, c++filt should be stripping the
leading underscore by default. Introduce the binutil compatible "-n"
option to control strip-undercore behaivor together with the existing
"-_" option and fallback to system default if none of them are set.

rdar://problem/57173514

Reviewers: compnerd, erik.pilkington, dexonsmith, mattd

Reviewed By: compnerd, erik.pilkington

Subscribers: jkorous, ributzka, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70250