Report sort phase progress in parallel btree build

We were already reporting it, but only after the parallel workers were
finished, which is visibly much later than what happens in a serial
build.

With this change we report it when the leader starts its own sort phase
when participating in the build (the normal case).  Now this might
happen a little later than when the workers start their sorting phases,
but a) communicating the actual phase start from workers is likely to be
a hassle, and b) the sort phase start is pretty fuzzy anyway, since
sorting per se is actually initiated by tuplesort.c internally earlier
than tuplesort_performsort() is called.

Backpatch to pg12, where the progress reporting code for CREATE INDEX
went in.

Reported-by: Tomas Vondra <tomas.vondra@enterprisedb.com>
Author: Matthias van de Meent <boekewurm+postgres@gmail.com>
Reviewed-by: Greg Nancarrow <gregn4422@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/1128176d-1eee-55d4-37ca-e63644422adb

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