lld-link: Only print demangled symbol names by default

This makes lld-link's output a bit more concise. Since most developers can't
read mangled names, this should make the output a bit easier to understand as
well. It also makes lld-link's output consistent with ld.lld's output.

(link.exe prints both demangled and mangled names; lld-link used to match
link.exe output but now no longer does.)

For people working on toolchains, add a `/demangle:no` flag that makes lld-link
print the mangled name instead of the demangled name. (If desired, people could
pipe that through `demumble -b` to get the old behavior of both demangled and
mangled output.)

Differential Revision: https://reviews.llvm.org/D58132

llvm-svn: 355878