tsan: lock ScopedErrorReportLock around fork

Currently we don't lock ScopedErrorReportLock around fork
and it mostly works becuase tsan has own report_mtx that
is locked around fork and tsan reports.
However, sanitizer_common code prints some own reports
which are not protected by tsan's report_mtx. So it's better
to lock ScopedErrorReportLock explicitly.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D106048