Install Roslyn binaries, including msbuild tasks to msbuild/Roslyn (#5251)

Install Roslyn binaries, including msbuild tasks to msbuild/Roslyn (#5251)

.. directory, which is `$mono_prefix/lib/mono/msbuild/15.0/bin/Roslyn/`

Mono installs `csc.exe` but the corresponding
`Microsoft.Build.Tasks.CodeAnalysis.dll` required for supporting msbuild C#
projects is installed by msbuild. But these two can get out of sync,
like they did and had to be worked around in
https://github.com/mono/msbuild/pull/19 .

Instead, we now install all the relevant roslyn bits (C# and VB) from the same
source. Only the msbuild specific files are copied to the Roslyn folder.
VBCSCompiler, csi and their dependencies are symlinked from `lib/mono/4.5`.

Also:

- csi.exe is included since the tool is surfaced as an msbuild task,
  and that requires the *Scripting.dll assemblies.
- VBCSCompiler* - shared compiler is installed
- vbc.exe doesn't work on mono currently, so skipping that
- And `Microsoft.DiaSymReader.Native.{amd64,x86}.dll` are dropped.