Validate ClangWorkspaceSettings before preferring it over unknown WorkspaceSettingsChanges

Basically everything will be decoded as ClangWorkspaceSettings, even if it has nothing to
do wit hit, yielding for example:
```
clangd(LanguageServerProtocol.ClangWorkspaceSettings(compilationDatabasePath: nil, compilationDatabaseChanges: nil))
```
This breaks at least my case of using WorkspaceSettingsChange. This patch adds a - maybe bandaid - fix for that.

Furthermore I removed one Test, as it - AFAIU - broke the invariant that none of the attributes
of ClangWorkspaceSettings may be nil.

Author: JCWasmx86

Sources/LanguageServerProtocol/SupportTypes/WorkspaceSettings.swift

@@ -19,10 +19,7 @@ public enum WorkspaceSettingsChange: Codable, Hashable {
   case unknown(LSPAny)
 
   public init(from decoder: Decoder) throws {
-    // FIXME: doing trial deserialization only works if we have at least one non-optional unique
-    // key, which we don't yet.  For now, assume that if we add another kind of workspace settings
-    // it will rectify this issue.
-    if let settings = try? ClangWorkspaceSettings(from: decoder) {
+    if let settings = try? ClangWorkspaceSettings(from: decoder), settings.isValid {
       self = .clangd(settings)
     } else {
       let settings = try LSPAny(from: decoder)
@@ -59,6 +56,13 @@ public struct ClangWorkspaceSettings: Codable, Hashable {
     self.compilationDatabasePath = compilationDatabasePath
     self.compilationDatabaseChanges = compilationDatabaseChanges
   }
+
+  var isValid: Bool {
+    switch (compilationDatabasePath, compilationDatabaseChanges) {
+      case (nil, .some), (.some, nil): return true
+      default: return false
+    }
+  }
 }
 
 /// A single compile command for use in a clangd workspace settings.

Tests/LanguageServerProtocolTests/CodingTests.swift

@@ -173,10 +173,6 @@ final class CodingTests: XCTestCase {
         """
     )
 
-    checkCoding(
-      WorkspaceSettingsChange.clangd(ClangWorkspaceSettings(compilationDatabasePath: nil)),
-      json: "{\n\n}"
-    )
     checkCoding(
       WorkspaceSettingsChange.clangd(ClangWorkspaceSettings(compilationDatabasePath: "foo")),
       json: """
@@ -186,14 +182,13 @@ final class CodingTests: XCTestCase {
         """
     )
 
-    // FIXME: should probably be "unknown"; see comment in WorkspaceSettingsChange decoder.
     checkDecoding(
       json: """
         {
           "hi": "there"
         }
         """,
-      expected: WorkspaceSettingsChange.clangd(ClangWorkspaceSettings(compilationDatabasePath: nil))
+      expected: WorkspaceSettingsChange.unknown(LSPAny.dictionary(["hi": LSPAny.string("there")]))
     )
 
     // experimental can be anything