Unify configuration of macro expansion.

    // expand macros as per normal
    -Ymacro-expand:normal

    // don't expand the macro, takes the place of -Ymacro-no-expand
    -Ymacro-expand:none

    // expand macros to compute type and emit warnings,
    // but retain expandee
    -Ymacro-expand:discard

This leaves to door ajar for a new option:

    // Don't expand blackbox macros; expand whitebox
    // but retain expandee
    -Ymacro-expand:discard-whitebox-only

The existing test for SI-6812 has been duplicated. One copy exercises
the now-deprecated -Ymacro-no-expand, and the other uses the new
option.