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

We see some crashes presumably caused by invalid 'offset' returned from
macro plugins. Although we haven't been able to find a reproducer, guard
not to generate invalid source location by checking 'offfset' is within
the buffer length.

rdar://125625879
(cherry picked from commit 9d336a0)

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)
   }