Pass parameters as implode($glue, $pieces); reverse order is deprecated

This is quite simple: implode($glue, $parts) is not going to cause issues with any supported PHP version.

Triage.

First priority before this:

To increase our confidence in the test results for the above php 7.4 compatibility issue, these two others should go in first so we can re-queue php 7.3 and php 5.3 tests.
#3047844: [Regression] Tests fail on PHP 5.3
#3025335: session_id() cannot be changed after session is started

8.x fixed

Re-queued testing with php 7.4 as per comment #5 with reference issues now closed.

When patch in comment #2 is applied it resolves the deprecated implode function with php 7.4 but exposes warnings as described in #3084953: Trying to access array offset on value of type null in _drupal_build_css_path() where there is a working patch.

Looking at the docs for implode() it seems it's not necessary to add an empty $glue parameter.

Looking at D8 after #3104421: PHP 7.4 deprecated reverse order of glue and pieces in implode, there are still examples of calls with just the $pieces param such as:

core/modules/filter/filter.module:580:    $text = implode($chunks);

[edit: removed some examples which were not PHP's native implode() function]

So I think we can remove the last few changes in the patch where there's only one param.

Great, thank you!

I've not been able to find any other calls with the params the wrong way around in D7 core.

This LGTM for commit; ready for final review.

RTBM - Ready to be merged, let's get this in.

Thanks everybody!