Add helper demangle function for 3rd party tools

[ryuukk]

Example of tools that'd benefit it:

https://github.com/KDAB/hotspot

Their build script is unnecessarily complex: https://github.com/KDAB/hotspot/blob/6f9f958c4ff68b3c94ccca0fa3c2d86c46c9185f/scripts/appimage/Dockerfile#L88-L98

Instead, i propose to add this function into druntime, so the tool can directly consume it

Making the whole process much easier:

-    loadDemangleLib(QStringLiteral("d_demangle"), "demangle_symbol", QByteArrayLiteral("_D"));
+    loadDemangleLib(QStringLiteral("phobos2"), "__d_demangle", QByteArrayLiteral("_D"));

https://github.com/KDAB/perfparser/blob/f38363858a56e1dc3b33b998e5a0f6c7046b8993/app/demangler.cpp#L40

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Testing this PR locally

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dub run digger -- build "master + dmd#16295"

[dkorpel]

Their build script is unnecessarily complex

The linked to script clones a repository containing a thin C-api wrapper around std.demangle. I think it's a good idea to provide a C-api directly in druntime, it just needs a test because I don't think it works in its current form.

[drpriver]

This is an out-of-bounds write if bufferLength == demangled.length