Mono's fork of msbuild uses a `$(_DebugFileExt)` property to specify
the debug file extension (.pdb/.mdb) to use. It defaults to .pdb. So,
with XI/XM, .mdb files don't get copied to the output folder.
We also add a `$(FscDebugFileExt)` property, which allows our default of
`.mdb` to be overridden.
But the `$(_DebugFileExt)` support is not in upstream msbuild yet. So,
we can't ask the F# upstream to add it. Hence, this is being added to
our FSharp target files. Once, all this is upstream, we can remove the
overrides from our files.
Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=51148
Ankit Jain
nosami
Rolf Bjarne Kvinge