[DiagnosticsBridge] Treat empty category name as nil

Default value for `Category` for serialization purposes is
an empty string but it should be handled as `nil` while
bridging because category name cannot be empty when present.

Author: xedin

lib/ASTGen/Sources/ASTGen/DiagnosticsBridge.swift

@@ -339,13 +339,20 @@ public func addQueuedDiagnostic(
     }
   }
 
-  let category: DiagnosticCategory? = categoryName.map { categoryNamePtr in
+  let category: DiagnosticCategory? = categoryName.flatMap { categoryNamePtr in
     let categoryNameBuffer = UnsafeBufferPointer(
       start: categoryNamePtr,
       count: categoryLength
     )
     let categoryName = String(decoding: categoryNameBuffer, as: UTF8.self)
 
+    // If the data comes from serialized diagnostics, it's possible that
+    // the category name is empty because StringRef() is serialized into
+    // an empty string.
+    guard !categoryName.isEmpty else {
+      return nil
+    }
+
     let documentationURL = documentationPath.map { documentationPathPtr in
       let documentationPathBuffer = UnsafeBufferPointer(
         start: documentationPathPtr,

lib/Frontend/CachedDiagnostics.cpp

@@ -617,7 +617,7 @@ llvm::Error DiagnosticSerializer::deserializeDiagnosticInfo(
                                   Kind,
                                   Info.FormatString,
                                   {},
-                                  Info.Category.empty() ? StringRef() : Info.Category,
+                                  Info.Category,
                                   *BICD,
                                   ChildDiagPtrs,
                                   Ranges,