[Demangle] Extract nonMicrosoftDemangle from llvm::demangle

Introduce a new demangling function that supports symbols using Itanium
mangling and Rust v0 mangling, and is expected in the near future to
include support for D mangling as well.

Unlike llvm::demangle, the function does not accept extra underscore
decoration. The callers generally know exactly when symbols should
include the extra decoration and so they should be responsible for
stripping it.

Functionally the only intended change is to allow demangling Rust
symbols with an extra underscore decoration through llvm::demangle,
which matches the existing behaviour for Itanium symbols.

Reviewed By: dblaikie, jhenderson

Part of https://reviews.llvm.org/D110664