[llvm] [CAS] Add MappedFileRegionArena (PR #114099)

[Rainer Orth via llvm-commits]

rorth wrote:

> > I will disable solaris as well. I need to read some documentation about that. It seems the OS is running out of lock resources? I don't think too much locks are used so I will workaround while I investigate.

Thanks, though the bots will take a while to catch up.

> @rorth Is Solaris using NFS?

Not on the bots: everything is local there.

I've meanwhile run a local build which reproduces the errors.  In that case, the source tree comes per NFS while the build directory is local.  However, I cannot yet make sense of what I'm seeing there: running
```
 ./unittests/CAS/CASTests --gtest_filter=CASProgramTest.MappedFileRegionArenaTest
```
under `truss` (the Solaris syscall tracer), I don't see the `ENOLCK` error that's present in the testcase output:
```
[ RUN      ] CASProgramTest.MappedFileRegionArenaTest
/vol/llvm/src/llvm-project/local/llvm/unittests/CAS/ProgramTest.cpp:97: Failure
Value of: llvm::detail::TakeError(MappedFileRegionArena::create(Path, 10 * 1024 * 1024, 0, emptyConstructor) .moveInto(Alloc))
Expected: succeeded
  Actual: failed  ('/var/tmp/MappedFileRegionArena-52c8d7/allocation-file.shared': No record locks available)
```
but I don't see it returned/set anywhere in the LLVM sources either.

https://github.com/llvm/llvm-project/pull/114099