[lldb][test] Fix D lang mangling test on Windows (#94196)

On Windows the function does not have a symbol associated with it:
Function: id = {0x000001c9}, name = "_Dfunction", range =
[0x0000000140001000-0x0000000140001004)
      LineEntry: <...>

Whereas it does on Linux:
Function: id = {0x00000023}, name = "_Dfunction", range =
[0x0000000000000734-0x0000000000000738)
      LineEntry: <...>
Symbol: id = {0x00000058}, range =
[0x0000000000000734-0x0000000000000738), name="_Dfunction"

This means that frame.symbol is not valid on Windows.

However, frame.function is valid and it also has a "mangled" attribute.

So I've updated the test to check the symbol if we've got it, and the
function always.

In both cases we check that mangled is empty (meaning it has not been
treated as mangled) and that the display name matches the original
symbol name.