Xcode: Fix rebuild with multiple custom command outputs (#15116)

The Xcode generator uses Makefiles under a run-script build-phase to
drive custom commands.  Fix the generated makefiles for custom commands
with multiple outputs to list all the outputs on the left hand side of
the build rule.  This is much simpler and more reliable than the old
multiple-output-pair infrastructure.
Makefile: Fix rebuild with multiple custom command outputs (#15116)

Fix the generated makefiles for custom commands with multiple outputs to
list all the outputs on the left hand side of the build rule.  This is
much simpler and more reliable than the old multiple-output-pair
infrastructure.
Tests: Cover rebuild with multiple custom command outputs (#15116)

Extend the BuildDepends test with a case covering multiple custom
command outputs with the second one consumed by another rule.  With the
old "multiple output pair" infrastructure used in the Makefile and Xcode
generators this did not work.  Now that it is fixed, test the case
explicitly.
Merge topic 'custom-command-multiple-outputs' into next

65ea5eb7 Tests: Cover rebuild with multiple custom command outputs (#15116)
644b4688 Makefile: Fix rebuild with multiple custom command outputs (#15116)
8a4c6d2d Xcode: Fix rebuild with multiple custom command outputs (#15116)