[NFC][clang] Update ubsan-trap-merge.c test to show absence of nomerg…

…e in non-trap mode (llvm#119280)

This shows that ubsan handlers do not have nomerge attributes in
non-trap mode, even if -ubsan-unique-trap is enabled.

0d15d46 attaches nomerge but only for
trap mode.

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Co-authored-by: Vitaly Buka <[email protected]>