Warn if LOCKTAG_TUPLE is held at commit, under debug_assertions.

Warn if LOCKTAG_TUPLE is held at commit, under debug_assertions.

The current use always releases this locktag.  A planned use will
continue that intent.  It will involve more areas of code, making unlock
omissions easier.  Warn under debug_assertions, like we do for various
resource leaks.  Back-patch to v12 (all supported versions), the plan
for the commit of the new use.

Reviewed by Heikki Linnakangas.

Discussion: https://postgr.es/m/20240512232923.aa.nmisch@google.com