gdb/dwarf: read correct rnglist/loclist header in read_{rng,loc}list_index

gdb/dwarf: read correct rnglist/loclist header in read_{rng,loc}list_index

When loading the binary from PR 26813 in GDB, we get:

    DW_FORM_rnglistx index pointing outside of .debug_rnglists offset array [in module /home/simark/build/binutils-gdb/gdb/MagicPurse]

... and the symbols fail to load.

In read_rnglist_index and read_loclist_index, we read the header
(documented in sections 7.28 and 7.29 of DWARF 5) of the CU's
contribution to the .debug_rnglists / .debug_loclists sections to
validate that the index we want to read makes sense.  However, we always
read the header at the beginning of the section, rather than the header
for the contribution from which we want to read the index.

To illustrate, here's what the binary from PR 26813 contains.  There are
two compile units:

0x0000000c: DW_TAG_compile_unit 1
              DW_AT_ranges [DW_FORM_rnglistx]: 0x0
              DW_AT_rnglists_base [DW_FORM_sec_offset]: 0xC

0x00003ec9: DW_TAG_compile_unit 2
              DW_AT_ranges [DW_FORM_rnglistx]: 0xB
              DW_AT_rnglists_base [DW_FORM_sec_offset]: 0x85

The layout of the .debug_rnglists is the following:

    [0x00, 0x0B]: header for CU 1's contribution
    [0x0C, 0x0F]: list of offsets for CU 1 (1 element)
    [0x10, 0x78]: range lists data for CU 1

    [0x79, 0x84]: header for CU 2's contribution
    [0x85, 0xB4]: list of offsets for CU 2 (12 elements)
    [0xB5, 0xBD7]: range lists data for CU 2

The DW_AT_rnglists_base attrbute points to the beginning of the list of
offsets for that CU, relative to the start of the .debug_rnglists
section.  That's right after the header for that contribution.

When we try to read the DW_AT_ranges attribute for CU 2,
read_rnglist_index reads the header for CU 1 instead of the one for CU
2.  Since there's only one element in CU 1's offset list, it believes
(wrongfully) that the index 0xB is out of range.

Fix it by reading the header just before where DW_AT_rnglists_base
points to.  With this patch, I am able to load GDB built with clang-11
and -gdwarf-5 in itself, with and without -readnow.

gdb/ChangeLog:

	PR gdb/26813
	* dwarf2/read.c (read_loclists_rnglists_header): Add
	header_offset parameter and use it.
	(read_loclist_index): Read header of the current contribution,
	not the one at the beginning of the section.
	(read_rnglist_index): Likewise.

Change-Id: Ie53ff8251af8c1556f0a83a31aa8572044b79e3d