Support for UNDNAME_NO_ARGUMENTS

[garyttierney]

Using the NO_ARGUMENTS flag with UnDecorateSymbolName allows the user to demangle a plain type (e.g. RTTI name) with no symbol. This is useful for demangling names produced by type_id().

The MSDN documentation implies this would be used to avoid demangling function parameters, but the WINE implementation (and in my experience) disagrees: https://github.com/wine-mirror/wine/blob/master/dlls/msvcrt/undname.c#L1458-L1469.

The easy way forward is simply to shortcut to read_var_type() in parse if that flag is present and return no Symbol. I've tested a hack that does something like this to make sure it works:

    fn parse(&mut self) -> Result<ParseResult<'a>> {
        // MSVC-style mangled symbols must start with b'?'.
        if !self.consume(b"?") {
            return Err(self.fail("does not start with b'?'"));
        }

        if self.type_only {
            let ty = self.read_var_type(StorageClass::empty())?;
            return Ok(ParseResult {
                // TODO: making the `Symbol` an `Option` would be a breaking change, how should we handle this?
                symbol: Symbol {
                    name: Name::Discriminator(0),
                    scope: NameSequence { names: Vec::new() }
                },
                symbol_type: ty,
            });
        }

And adjust the serializer to only emit the type. I have a work in progress patch and will open a PR shortly, but the current issues I have are how to handle the optional Symbol and any information about NO_ARGUMENTS that I might be missing beyond my own experience/the WINE implementation.

[garyttierney]

Examples: https://godbolt.org/z/YMEWq3q5x

??_R0?AV?$C@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@@@8 DQ FLAT:??_7type_info@@6B@ ; C<std::basic_string<char,std::char_traits<char>,std::allocator<char> > > `RTTI Type Descriptor'
        DQ      0000000000000000H
        DB      '.?AV?$C@V?$basic_string@DU?$char_traits@D@std@@V?$alloca'
        DB      'tor@D@2@@std@@@@', 00H
??_R0?AVB@0@@8 DQ FLAT:??_7type_info@@6B@             ; B::B `RTTI Type Descriptor'
        DQ      0000000000000000H
        DB      '.?AVB@0@', 00H
??_R0?AVA@@@8 DQ FLAT:??_7type_info@@6B@                ; A `RTTI Type Descriptor'
        DQ      0000000000000000H
        DB      '.?AVA@@', 00H