GCC recently added a new warning -Wformat-diag which does a lot of
    rigorous checks on GCC diagnostic messages.  This produces a number of
    unnecessary diagnostics on gofrontend diagnostic output, such as
    
    ../../trunk/gcc/go/gofrontend/escape.cc: In member function ‘virtual int Escape_analysis_assign::statement(Block*, size_t*, Statement*)’:
    ../../trunk/gcc/go/gofrontend/escape.cc:1336:33: warning: spurious leading punctuation sequence ‘[’ in format [-Wformat-diag]
     1336 |       go_inform(s->location(), "[%d] %s esc: %s",
          |                                 ^
    
    ../../trunk/gcc/go/gofrontend/escape.cc: In member function ‘void Escape_analysis_assign::call(Call_expression*)’:
    ../../trunk/gcc/go/gofrontend/escape.cc:1964:17: warning: unquoted operator ‘::’ in format [-Wformat-diag]
     1964 |         "esccall:: indirect call <- %s, untracked",
          |                 ^~
    
    ../../trunk/gcc/go/gofrontend/escape.cc:1964:34: warning: unbalanced punctuation character ‘<’ in format [-Wformat-diag]
     1964 |         "esccall:: indirect call <- %s, untracked",
          |                                  ^
    
    Avoid these messages by adding a new function go_debug that uses only
    printf formatting, not GCC diagnostic formatting, and change all the
    optimization debugging messages to use it.  None of the debugging
    messages used the GCC diagnostic formatting specifiers anyhow.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/183437

From-SVN: r272607