Changes

Reapply: [MachO] Add a test for detecting reserved unit length.
The test in the origin patch did not create a __debug_str section. An
UBSan check triggered when the corresponding pointer was dereferenced.
Differential Revision: https://reviews.llvm.org/D72695
This reapplies fcc08aa835de1e0c1f3e7e479917575e55433b68 which was
reverted in b16f82ad3b095070729cde2873bd3a2e8ab35c0e.