gdb-gdb.py.in: Fix error when printing range type

gdb-gdb.py.in: Fix error when printing range type

I noticed that trying to print the contents of a struct main_type
would fail when the type was a TYPE_CODE_RANGE:

    (gdb) p *type.main_type
    $1 = Python Exception <class 'gdb.error'> There is no member named low_undefined.:

And indeed, Python is right, fields "low_undefined" has been removed
from struct range_bounds back in ... 2014! It was done when we introduced
dynamic bounds handling. This patch fixes gdb-gdb.py.in according to
the new structure.

gdb/ChangeLog:

	* gdb-gdb.py.in (StructMainTypePrettyPrinter.bound_img): New method.
	(StructMainTypePrettyPrinter.bounds_img): Use new "bound_img"
	method to compute the bounds of range types. Also print "[evaluated]"
	if the bounds' values come from a dynamic evaluation.