Revert "Apply modernize-use-default to compiler-rt."

This reverts commit r250823.

Replacing at least some of empty
constructors with "= default" variants is a semantical change which we
don't want. E.g. __tsan::ClockBlock contains a union of large arrays,
and it's critical for correctness and performance that we don't memset()
these arrays in the constructor.

llvm-svn: 251717