Fix decoding of multi-dimensional constrained packed arrays

Printing a multi-dimensional constrained packed array in Ada would not
show the correct values.  The bug here is that, when decoding the type
of such an array, only the innermost dimension's element bitsize would
be correct.  For outer dimensions, the bitsize must account for the
size of each sub-array, but this was not done.

This patch fixes the problem by arranging to compute these sizes after
decoding the array type.  I've included a bit more test case than is
strictly necessary -- the current test here was derived from an
internal test, and this patch brings the two into sync.

gdb/ChangeLog
2020-11-04  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (recursively_update_array_bitsize): New function.
	(decode_constrained_packed_array_type): Call it.

gdb/testsuite/ChangeLog
2020-11-04  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/enum_idx_packed.exp: Add tests.
	* gdb.ada/enum_idx_packed/foo.adb: Add variables.
	* gdb.ada/enum_idx_packed/pck.adb: Add functions.
	* gdb.ada/enum_idx_packed/pck.ads: Add types, function
	declarations.