[ELF] Allow --noinhibit-exec to produce corrupted executable with relocation overflow

Summary:
When --noinhibit-exec is used, ld.bfd/gold emit errors but allow to produce corrupted executable, which is handy for debugging purpose. lld's --noinhibit-exec has a different meaning and changes some errors to warnings. This patch replaces "error" with "errorOrWarn" to exploit that property.

We may revisit this: if we should keep them as errors (as ld.bfd/gold do) but allow to produce a (corrupted) executable.

Reviewers: ruiu, grimar, espindola

Reviewed By: grimar

Subscribers: Timmmm, jhenderson, emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D54651

llvm-svn: 347327