[llvm] [llvm][Support] Only enable backtrace test when it's enabled (PR #123852)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

lld and clang already do this, so can we make it the same as they do? I.e. use ENABLE_BACKTRACE instead of LLVM_ENABLE_BACKTRACE (though really, the exported name should be renamed to have the LLVM prefix...) and name the feature `backtrace`? lld/clang also do the feature addition in lit.cfg.py and only assign the value from cmake in lit.site.cfg.py.in.

https://github.com/llvm/llvm-project/pull/123852