Replace libcxxabi demangler with demumble's LLVM and Swift demanglers

[rjmansfield]

This replaces the old libcxxabi-based C++ demangler with demanglers
from the demumble project, which includes LLVM's demangler for C++
Itanium, Rust and Microsoft symbols, as well as the Swift demangler.

This improves Bloaty's C++ demangling support, as well as adds support
for demangling Swift, Rust and Microsoft/MSVC symbols.

Fixes #110
Fixes #394

[rjmansfield]

I have a PR open ( nico/demumble#40) to update the demanglers in demumble as they're about a year out of date, and don't have the latest llvm and swift changes. However, even if Bloaty adopts demumble at 10e00fb, it will resolve the issues and be an overall improvement in demangling. Then in the future, if demumble is updated, it should be relatively straight forward to update the submodule.

[EricRahm]

Looks good, just a few questions. Thanks again for the thorough tests!

[EricRahm]

I have a PR open ( nico/demumble#40) to update the demanglers in demumble as they're about a year out of date, and don't have the latest llvm and swift changes. However, even if Bloaty adopts demumble at 10e00fb, it will resolve the issues and be an overall improvement in demangling. Then in the future, if demumble is updated, it should be relatively straight forward to update the submodule.

Landing as is and the updating the submodule makes sense to me, I'll add a note to that PR that this would be useful for Bloaty and the Fuchsia project.

[EricRahm]

Thanks for the improvements!