Merge pull request swiftlang#72771 from rintaro/6.0-macros-bridgedloc-rdar125625879

[6.0][ASTGen/Macros] Ensure 'offset' from plugin response is valid

Author: rintaro

lib/ASTGen/Sources/ASTGen/PluginHost.swift

@@ -349,6 +349,10 @@ class PluginDiagnosticsEngine {
     guard let bufferBaseAddress = exportedSourceFile.pointee.buffer.baseAddress else {
       return nil
     }
+    // Ensure 'offset' is within the buffer.
+    guard offset <= exportedSourceFile.pointee.buffer.count else {
+      return nil
+    }
     return BridgedSourceLoc(raw: bufferBaseAddress).advanced(by: offset)
   }