Handle double leading underscores on Mac

[kalyanac]

Filter() cannot handle double leading underscore on Mac OS X.

Here is a sample code and output:

package main

import (
"github.com/ianlancetaylor/demangle"
"fmt"
)


func main(){
	name := "__ZNSt3__111char_traitsIcE11eq_int_typeEii"
	options := []demangle.Option{demangle.NoParams, demangle.NoTemplateParams}
	fmt.Println(demangle.Filter(name, options...))
	fmt.Println(demangle.Filter(name[1:], options...))

}

Output:

$> go run test.go
__ZNSt3__111char_traitsIcE11eq_int_typeEii
std::__1::char_traits::eq_int_type

[minux]

it's because macOS prefix every symbol with an extra underscore. I think it's the caller's responsibility to remove it before passing it to Filter. For example, c++filt on macOS also doesn't handle the example unless the leading underscore is stripped.

…

[kalyanac]

Yes, stripping it easy but it's fairly straight forward to detect the platform and handle the leading underscore. It makes the use of the package easier and returns the expected results.

[minux]

I guess adding a StripLeadingUnderscore Option might be OK, but stripping it unconditionally might be undesirable as the underscored version really is not C++ mangled name defined by the IA64 ABI (what if there is another platform that adds another prefix or even suffix to standard mangled name?)