[MS Demangler] NFC - Remove state from Demangler class.

We need to be able to initiate a nested demangling from inside
of an "outer" demangling.  These need to be able to share some
state, such as back-references.  As a result, we can't store
things like the output stream or the mangled name in the Demangler
class, since each demangling will have different values.  So
remove this state and pass it through the necessary methods.

llvm-svn: 338219