[demangler] Improve ->* & .* demangling

The demangler treats ->* as a BinaryExpr, but .* as a MemberExpr.
That's inconsistent.  This makes the former a MemberExpr too.
However, in order to not regress the paren output, MemberExpr::print
is modified to parenthesize the MemberExpr if the operator ends with
'*'.  Printing is affected thusly:

Before:
    obj.member
    obj->member
    obj.*member
    (obj) ->* (member)

After:
   obj.member   # Unchanged
   obj->member  # Unchanged
   obj.*(member)  # Added paren member operand
   obj->*(member) # Removed paren on object operand, less whitespace

The right solution to the paren problem is to add some notion of
precedence (and associativity) to Nodes, but that's a larger change
that would become simpler once the refactoring I'm doing is completed.

FWIW, binutils' demangler's paren algorithm has a small idea of
precedence, and will generally not emit parens when the operand is
unary.

Reviewed By: bruno

Differential Revision: https://reviews.llvm.org/D118486