[llvm] [CAS] Add OnDiskDataAllocator (PR #161264)

[Nico Weber via llvm-commits]

nico wrote:

This seems to not build:

http://45.33.8.238/linux/179183/step_3.txt

```
../../llvm/lib/CAS/OnDiskDataAllocator.cpp:212:31: error: no member named 'pointer' in 'llvm::cas::OnDiskDataAllocator'
  212 | Expected<OnDiskDataAllocator::pointer>
      |          ~~~~~~~~~~~~~~~~~~~~~^
../../llvm/lib/CAS/OnDiskDataAllocator.cpp:214:10: error: no viable conversion from returned value of type 'Error' to function return type 'int'
  214 |   return createStringError(make_error_code(std::errc::not_supported),
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  215 |                            "OnDiskDataAllocator is not supported");
      |                            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../llvm/lib/CAS/OnDiskDataAllocator.cpp:218:34: error: out-of-line definition of 'beginData' does not match any declaration in 'llvm::cas::OnDiskDataAllocator'
  218 | const char *OnDiskDataAllocator::beginData(FileOffset Offset) const {
      |                                  ^~~~~~~~~
3 errors generated.
```

Please take a look and revert for now if it takes a while to fix.

https://github.com/llvm/llvm-project/pull/161264