Add script to deploy just-built MSBuild (#4208)

Example usage (elevated command prompt):

```
powershell -file S:\msbuild\scripts\Deploy-MSBuild.ps1 -destination "C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\MSBuild\Current\Bin" -configuration Debug
```

scripts/Deploy-MSBuild.ps1

@@ -0,0 +1,47 @@
+[CmdletBinding(PositionalBinding=$false)]
+Param(
+  [Parameter(Mandatory = $true)]
+  [string] $destination,
+  [ValidateSet('Debug','Release')]
+  [string] $configuration = "Debug"
+)
+
+function Copy-WithBackup ($origin) {
+    $destinationPath = Join-Path -Path $destination -ChildPath (Split-Path $origin -leaf)
+
+    if (Test-Path $destinationPath -PathType Leaf) {
+        # Back up previous copy of the file
+        Copy-Item $destinationPath $BackupFolder -ErrorAction Stop
+    }
+
+    Copy-Item $origin $destinationPath -ErrorAction Stop
+}
+
+# TODO: find destination in PATH if not specified
+
+# TODO: identify processes likely to be using target MSBuild and warn/offer to kill
+
+# TODO: find most-recently-built MSBuild and make it default $configuration
+
+$BackupFolder = New-Item (Join-Path $destination -ChildPath "Backup-$(Get-Date -Format FileDateTime)") -itemType directory -ErrorAction Stop
+
+Write-Verbose "Copying $configuration MSBuild to $destination"
+Write-Host "Existing MSBuild assemblies backed up to $BackupFolder"
+
+$filesToCopyToBin = @(
+    "artifacts\bin\MSBuild.Bootstrap\$configuration\net472\MSBuild.exe"
+    "artifacts\bin\MSBuild.Bootstrap\$configuration\net472\Microsoft.Build.dll"
+    "artifacts\bin\Microsoft.Build.Conversion\$configuration\net472\Microsoft.Build.Conversion.Core.dll"
+    "artifacts\bin\Microsoft.Build.Engine\$configuration\net472\Microsoft.Build.Engine.dll"
+    "artifacts\bin\MSBuild.Bootstrap\$configuration\net472\Microsoft.Build.Framework.dll"
+    "artifacts\bin\MSBuild.Bootstrap\$configuration\net472\Microsoft.Build.Tasks.Core.dll"
+    "artifacts\bin\MSBuild.Bootstrap\$configuration\net472\Microsoft.Build.Utilities.Core.dll"
+    "artifacts\bin\MSBuildTaskHost\$configuration\net35\MSBuildTaskHost.exe"
+    "artifacts\bin\MSBuildTaskHost\$configuration\net35\MSBuildTaskHost.pdb")
+
+foreach ($file in $filesToCopyToBin) {
+    Copy-WithBackup $([IO.Path]::Combine($PSScriptRoot, "..", $file))
+}
+
+Write-Host -ForegroundColor Green "Copy succeeded"
+Write-Verbose "Run $destination\MSBuild.exe"