[BPF] Restrict preserve_access_index attribute to C only

This patch is a follow-up for commit 4e2ce228
  [BPF] Add preserve_access_index attribute for record definition
to restrict attribute for C only. A new test case is added
to check for this restriction.

Additional code polishing is done based on
Aaron Ballman's suggestion in https://reviews.llvm.org/D69759/new/.

Differential Revision: https://reviews.llvm.org/D70257