GDB: Introduce limited array lengths while printing values

GDB: Introduce limited array lengths while printing values

This commit introduces the idea of loading only part of an array in
order to print it, what I call "limited length" arrays.

The motivation behind this work is to make it possible to print slices
of very large arrays, where very large means bigger than
`max-value-size'.

Consider this GDB session with the current GDB:

  (gdb) set max-value-size 100
  (gdb) p large_1d_array
  value requires 400 bytes, which is more than max-value-size
  (gdb) p -elements 10 -- large_1d_array
  value requires 400 bytes, which is more than max-value-size

notice that the request to print 10 elements still fails, even though 10
elements should be less than the max-value-size.  With a patched version
of GDB:

  (gdb) p -elements 10 -- large_1d_array
  $1 = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9...}

So now the print has succeeded.  It also has loaded `max-value-size'
worth of data into value history, so the recorded value can be accessed
consistently:

  (gdb) p -elements 10 -- $1
  $2 = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9...}
  (gdb) p $1
  $3 = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
    20, 21, 22, 23, 24, <unavailable> <repeats 75 times>}
  (gdb)

Accesses with other languages work similarly, although for Ada only
C-style [] array element/dimension accesses use history.  For both Ada
and Fortran () array element/dimension accesses go straight to the
inferior, bypassing the value history just as with C pointers.

Co-Authored-By: Maciej W. Rozycki <macro@embecosm.com>