[demangler] Initial support for the new Rust mangling scheme

Add a demangling support for a small subset of a new Rust mangling
scheme, with complete support planned as a follow up work.

Intergate Rust demangling into llvm-cxxfilt and use llvm-cxxfilt for
end-to-end testing. The new Rust mangling scheme uses "_R" as a prefix,
which makes it easy to disambiguate it from other mangling schemes.

The public API is modeled after __cxa_demangle / llvm::itaniumDemangle,
since potential candidates for further integration use those.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D101444