Log the ClangSharpPInvokeGenerator path and ensure the script doesn't fail for missing tools

[tannergooding]

These were found while testing dotnet/ClangSharp#235.

In particular, it is useful to log the resolved path for ClangSharpPInvokeGenerator so it can be asserted to be the expected one (ClangSharp's CI test should override the global tool, for example).

Likewise, Select-String does not output a string by default, it outputs a Microsoft.PowerShell.Commands.MatchInfo: https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.utility/select-string?view=powershell-7.1#outputs which does not have a Length property. It is expected that -Raw be passed if you want a string.

Additionally, if there is no match the result can be $null, which needs to be checked before getting the length.

[tannergooding]

CC. @sotteson1

[sotteson1]

Thanks @tannergooding!