[ClangModule] Fix decl-params-determinisim test after serialization change (#72572)

Fix decl-params-determinisim test after 48be81e1 packed some information
in the clang module. The test is to make sure the decls are appearing in
a strict ordering and it relies on check the correct field in the
bitcode format.

Add more explanation in the comments to help future updates when
serialization format affects this test.