Acts as a placeholder for no-debug-info. Implemented as nullptr.

Bug: b/148401179
Change-Id: I6cbfb227296d98befa6a7d2baa29128cb0c7bd29
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/42193
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>