Finalize each cooked index separately

After DWARF has been scanned, the cooked index code does a
"finalization" step in a worker thread.  This step combines all the
index entries into a single master list, canonicalizes C++ names, and
splits Ada names to synthesize package names.

While this step is run in the background, gdb will wait for the
results in some situations, and it turns out that this step can be
slow.  This is PR symtab/29105.

This can be sped up by parallelizing, at a small memory cost.  Now
each index is finalized on its own, in a worker thread.  The cost
comes from name canonicalization: if a given non-canonical name is
referred to by multiple indices, there will be N canonical copies (one
per index) rather than just one.

This requires changing the users of the index to iterate over multiple
results.  However, this is easily done by introducing a new "chained
range" class.

When run on gdb itself, the memory cost seems rather low -- on my
current machine, "maint space 1" reports no change due to the patch.

For performance testing, using "maint time 1" and "file" will not show
correct results.  That approach measures "time to next prompt", but
because the patch only affects background work, this shouldn't (and
doesn't) change.  Instead, a simple way to make gdb wait for the
results is to set a breakpoint.

Before:

    $ /bin/time -f%e ~/gdb/install/bin/gdb -nx -q -batch \
        -ex 'break main' /tmp/gdb
    Breakpoint 1 at 0x43ec30: file ../../binutils-gdb/gdb/gdb.c, line 28.
    2.00

After:

    $ /bin/time -f%e ./gdb/gdb -nx -q -batch \
        -ex 'break main' /tmp/gdb
    Breakpoint 1 at 0x43ec30: file ../../binutils-gdb/gdb/gdb.c, line 28.
    0.65

Regression tested on x86-64 Fedora 34.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29105

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