Follow-up to r217302: Don't crash on ~A::A in a postfix expr suffix followed by '<'.

This used to crash, complaining "ObjectType and scope specifier cannot coexist":

    struct A { } b = b.~A::A <int>;

The only other caller of ParseOptionalCXXScopeSpecifier() that passes in a
non-empty ObjectType clears the ObjectType of the scope specifier comes back
non-empty (see the tok::period case in Parser::ParsePostfixExpressionSuffix()),
so do that here too.

Found by SLi's bot.

llvm-svn: 227781