[Support] Use InitLLVM in llvm-stress, sancov and TableGen

This patch refactors the llvm tools namely, llvm-stress and sancov,
as well as the llvm TableGen utility, to use the new InitLLVM
interface which encapsulates PrettyStackTrace.

This is from https://reviews.llvm.org/D70702, but only for LLVM.

Reviewed-by: Kai

Differential Revision: https://reviews.llvm.org/D83484