reland "[DebugInfo] Support to emit debugInfo for extern variables"

Commit d77ae155
("[DebugInfo] Support to emit debugInfo for extern variables")
added deebugInfo for extern variables for BPF target.
The commit is reverted by 891e25b0
as the committed tests using %clang instead of %clang_cc1 causing
test failed in certain scenarios as reported by Reid Kleckner.

This patch fixed the tests by using %clang_cc1.

Differential Revision: https://reviews.llvm.org/D71818