[ELF] Support -plugin-opt=stats-file=

This flag is added by clang::driver::tools::addLTOOptions() and was causing
errors for me when building the llvm-test-suite repository with LTO and
-DTEST_SUITE_COLLECT_STATS=ON. This replaces the --stats-file= option
added in 1c04b52b since the flag is only
used for LTO and should therefore be in the -plugin-opt= namespace.

Additionally, this commit fixes the `REQUIRES: asserts` that was added in
948d0532: the feature was never defined in
the lld test suite so it effectively disabled the test.

Reviewed By: MaskRay, MTC

Differential Revision: https://reviews.llvm.org/D124105