[asan][ubsan][test] Add weak attributes for dyld Mach-O weak-def-coalescing

Apple's dynamic linker won't weak-def_coalesce from a file unless there is
at least one weak symbol in the compilation unit so  local __ubsan_on_report
never has the chance to override the weak one even though the dynamic linker
may see it first. This works around the issue by adding an unused weak symbol.
(Amended: Remove excessive clang-format artifacts)

rdar://95244261

Differential Revision: https://reviews.llvm.org/D127929