[LSP] Do not recompute syntax diagnostics if syntax has not changed

[dibarbet]

Currently we always use the project dependent checksum to determine if there are any changes that might impact diagnostics - https://github.com/dotnet/roslyn/blob/main/src/LanguageServer/Protocol/Handler/Diagnostics/AbstractPullDiagnosticHandler.cs#L165

The request from the client is that for syntax diagnostic sources, we use the document / syntax only checksum to determine if there have been changes. This avoids unnecessary recomputation of the syntax only diagnostics when there is a change in another file for example