[LTO] Always mark regular LTO units with EnableSplitLTOUnit=1
Regular LTO modules do not need LTO Unit splitting, only ThinLTO does
(they must be consistently split into regular and Thin units for
optimizations such as whole program devirtualization and lower type
tests). In order to avoid spurious errors from LTO when combining with
split ThinLTO modules, always set this flag for regular LTO modules.
Subscribers: mehdi_amini, Prazek, inglorion, steven_wu, dexonsmith, cfe-commits
Differential Revision: https://reviews.llvm.org/D65009