[Support] Factor out isCrash from throwIfCrash

This diff factors out the check "isCrash" from the static method "throwIfCrash".
This is a helper function that can be useful in debugging / analysis, in particular,
I'm planning to use it in the future patches for lld-fuzzer.

Test plan:
1/ ninja check-all
2/ export LLD_IN_TEST=5 ninja check-lld

Differential revision: https://reviews.llvm.org/D124414