[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 =
[0x0000000140

[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.

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[lldb][test] Add missing import in D lang mangling test

[lldb][test] Skip D lang mangling test on Windows

While the fix is reviewed.

[lldb] Improve identification of Dlang mangled names (#93881)

Reduce false positive identification of C names as Dlang mangled names. This happens
when a C function uses the prefix `_D`.

The [Dl

[lldb] Improve identification of Dlang mangled names (#93881)

Reduce false positive identification of C names as Dlang mangled names. This happens
when a C function uses the prefix `_D`.

The [Dlang ABI](https://dlang.org/spec/abi.html#name_mangling) shows that mangled names
have a length immediately following the `_D` prefix. This change checks for a digit
after the `_D` prefix, when identifying the mangling scheme of a symbol. This doesn't
prevent false positives entirely, but does make it less likely.

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