compiler: add new debugging helper function debug_go_type()

compiler: add new debugging helper function debug_go_type()

    Add a new debugging utility routine debug_go_type(), intended to
    display the contents of a Type object in a way useful to debugging
    a run of the compiler.  Prior to this the only useful alternative
    for debugging types was invoking the mangled_name() method, which
    has problems (for example, won't work on interface types prior
    to finalizing of methods).

    This is a "deep" dump, meaning that all types reachable from the
    type passed to debug_go_type() will be printed out. Example:

    (gdb) print debug_go_type(t1)
          T0        0x535f300  'net/http.Header' -> T1
          T1        0x535d3d0  map ['string' -> string] T4
          T2        0x5304bb0  'string' -> string
          T3        0x331f900  string
          T4        0x535d370  array [] 'string' -> string

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/166637

From-SVN: r269633