[Serialized diagnostics] Ignore unknown records. (#379)

To improve forward compatibility, ignore any unknown records
encountered during deserialization rather than throwing an error.
libclang has behaved this way forever, and we should do the same.

Author: DougGregor

Sources/TSCUtility/SerializedDiagnostics.swift

@@ -129,7 +129,8 @@ extension SerializedDiagnostics {
           }
 
         case .version, nil:
-          throw Error.unexpectedRecord
+          // Ignore unknown records, for future compatibility
+          continue
         }
       }
 

Tests/TSCUtilityTests/Inputs/string_init_ambig.dia

@@ -163,6 +163,16 @@ final class SerializedDiagnosticsTests: XCTestCase {
     XCTAssertEqual(diag.fixIts.count, 0)
   }
 
+  func testReadDiagsWithUnknownRecord() throws {
+    let serializedDiagnosticsPath = AbsolutePath(path: #file).parentDirectory
+        .appending(components: "Inputs", "string_init_ambig.dia")
+    let contents = try localFileSystem.readFileContents(serializedDiagnosticsPath)
+    let serializedDiags = try SerializedDiagnostics(bytes: contents)
+
+    XCTAssertEqual(serializedDiags.versionNumber, 2)
+    XCTAssertEqual(serializedDiags.diagnostics.count, 35)
+  }
+
   func testReadClangSerializedDiags() throws {
     let serializedDiagnosticsPath = AbsolutePath(path: #file).parentDirectory
         .appending(components: "Inputs", "clang.dia")