[MS Demangler] Properly handle backreferencing of special names.

Function template names are not stored in the backref table,
but non-template function names are.  The general pattern seems
to be that when you are demangling a symbol name, if the name
starts with '?' it does not go into the backreference table,
otherwise it does.  Note that this even handles the general case
of operator names (template or otherwise) not going into the
back-reference table, anonymous namespaces not going into the
backreference table, etc.

It's important that we apply this check *only* for the
unqualified portion of a name, and only for symbol names.
For example, this does not apply to type names (such as class
templates) and we need to make sure that these still do go
into the backref table.

Differential Revision: https://reviews.llvm.org/D50394

llvm-svn: 339211