Fixed check-lld msan after r274504 "[ELF] - Implemented --fatal-warnings option."

Bot failed:
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fast/builds/14361/steps/check-lld%20msan/logs/stdio

Fix:
Initialize Config->FatalWarnings with false. As it might be used once a bit earlier than its
initialization from command line arguments.

llvm-svn: 274507