Replace FSComp invocations in VS tooling with an FCS API (#6904)

Author: cartermp

src/fsharp/service/ServiceCompilerDiagnostics.fs

@@ -0,0 +1,14 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace FSharp.Compiler.SourceCodeServices
+
+type DiagnosticKind =
+    | AddIndexerDot
+    | ReplaceWithSuggestion of suggestion:string
+
+[<RequireQualifiedAccess>]
+module CompilerDiagnostics =
+    let getErrorMessage diagnosticKind =
+        match diagnosticKind with
+        | AddIndexerDot -> FSComp.SR.addIndexerDot()
+        | ReplaceWithSuggestion s -> FSComp.SR.replaceWithSuggestion(s)

src/fsharp/service/ServiceCompilerDiagnostics.fsi

@@ -0,0 +1,13 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace FSharp.Compiler.SourceCodeServices
+
+/// Supported kinds of diagnostics by this service.
+type DiagnosticKind =
+    | AddIndexerDot
+    | ReplaceWithSuggestion of suggestion:string
+
+/// Exposes compiler diagnostic error messages.
+module CompilerDiagnostics =
+    /// Given a DiagnosticKind, returns the string representing the error message for that diagnostic.
+    val getErrorMessage: diagnosticKind: DiagnosticKind -> string