Improve line info tracing, in particular when using lto.

Improve line info tracing, in particular when using lto.

With gcc 9 and --enable-lto, we now have spurious warnings telling
that the line information in the debug info has huge line numbers,
greater than the (valgrind) maximum of 2^20.

These spurious warnings make that all tests are failing.

This change modifies the tracing/debugging of the line info to:
  * disable by default the warning for line info greater than 2^20.
    When using -d, such warnings are however still shown (once).
  * allow to see all such warnings, when using at least -d -d -d -d