[gdb/symtab] Fix duplicate CUs in all_comp_units

When running test-case gdb.cp/cpexprs-debug-types.exp with target board
cc-with-debug-names on a system with gcc 12.1.1 (defaulting to dwarf 5), I
run into:
...
(gdb) file cpexprs-debug-types^M
Reading symbols from cpexprs-debug-types...^M
warning: Section .debug_aranges in cpexprs-debug-types has duplicate \
  debug_info_offset 0x0, ignoring .debug_aranges.^M
gdb/dwarf2/read.h:309: internal-error: set_length: \
  Assertion `m_length == length' failed.^M
...

The exec contains a .debug_names section, which gdb rejects due to
.debug_names containing a list of TUs, while the exec doesn't contain a
.debug_types section (which is what you'd expect for dwarf 4).

Gdb then falls back onto the cooked index, which calls create_all_comp_units
to create all_comp_units.  However, the failed index reading left some
elements in all_comp_units, so we end up with duplicates in all_comp_units,
which causes the misleading complaint and the assert.

Fix this by:
- asserting at the start of create_all_comp_units that all_comp_units is empty,
  as we do in create_cus_from_index and create_cus_from_debug_names, and
- cleaning up all_comp_units when failing in dwarf2_read_debug_names.

Add a similar cleanup in dwarf2_read_gdb_index.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29381

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