[Demangle][Rust] Parse dot suffix

Allow mangled names to include an arbitrary dot suffix, akin to vendor
specific suffix in Itanium mangling.

Primary motivation is a support for symbols renamed during ThinLTO
import / promotion (ThinLTO is the default configuration for optimized
builds in rustc).

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D104358