lld-link: print demangled symbol names for "undefined symbol" diagnostics

For this, add a few toString() calls when printing the "undefined symbol"
diagnostics; toString() already does demangling on Windows hosts.

Also make lld::demangleMSVC() (called by toString(Symbol*)) call LLVM's
microsoftDemangle() instead of UnDecorateSymbolName() so that it works on
non-Windows hosts – this makes both updating tests easier and provides a better
user experience for people doing cross-links.

This doesn't yet do the right thing for symbols starting with __imp_, but that
can be improved in a follow-up.

Differential Revision: https://reviews.llvm.org/D52104

llvm-svn: 342332