[libclc] Fix ctest failures after 7f3661128b1e: adjust external check and make shuffle helpers static (#160036)
* Replace call-site check with external declaration scan (grep declare)
to avoid false positives for not-inlined __clc_* functions.
* _clc_get_el* helpers are defined as inline in clc_shuffle2.cl, so they
have available_externally attribute. When they fail to inline they are
deleted by EliminateAvailableExternallyPass and become unresolved in
cedar-r600--.bc. Mark them static to resolve the issue.
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>