[gdb/symtab] Handle DW_AT_ranges with DW_FORM_sec_off in partial DIE

[gdb/symtab] Handle DW_AT_ranges with DW_FORM_sec_off in partial DIE

While looking into a failure in gdb.go/package.exp with gcc-11, I noticed that
gdb shows some complaints when loading the executable (also with gcc-10, where
the test-case passes):
...
$ gdb -batch -iex "set complaints 100" package.10 -ex start
During symbol reading: Attribute value is not a constant (DW_FORM_sec_offset)
Temporary breakpoint 1 at 0x402ae6: file gdb.go/package1.go, line 8.
During symbol reading: Attribute value is not a constant (DW_FORM_sec_offset)
During symbol reading: Invalid .debug_rnglists data (no base address)
...

Fix this by using as_unsigned () to read DW_AT_ranges in the partial DIE
reader, similar to how that is done in dwarf2_get_pc_bounds.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-01-25  Bernd Edlinger  <bernd.edlinger@hotmail.de>
	    Simon Marchi  <simon.marchi@polymtl.ca>
	    Tom de Vries  <tdevries@suse.de>

	* dwarf2/read.c (partial_die_info::read): Use as_unsigned () for
	DW_AT_ranges.

gdb/testsuite/ChangeLog:

2021-01-25  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/dw2-ranges-psym.exp (gdb_load_no_complaints): New proc.
	* lib/gdb.exp: Use gdb_load_no_complaints.