[CAS] Fix build failure from #114099 using gcc (#158697)
Fix a buildbot failure on some bots using gcc:
```
MappedFileRegionArena.cpp:275:10: error: could not convert ‘Result’ from ‘llvm::cas::MappedFileRegionArena’ to ‘llvm::Expected<llvm::cas::MappedFileRegionArena>’
```
This is caused by RVO not activated causing the type check error.
