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

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64-flang-aix` running on `ppc64-flang-aix-test` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/201/builds/6557

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (build-unified-tree) failure: build (failure)
...
34.387 [1/8/165] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/reduce.cpp.o
34.512 [1/7/166] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/tools.cpp.o
45.742 [1/6/167] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/character.cpp.o
61.689 [1/5/168] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/dot-product.cpp.o
191.201 [1/4/169] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/matmul-transpose.cpp.o
222.078 [1/3/170] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/extrema.cpp.o
243.625 [1/2/171] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/matmul.cpp.o
278.189 [1/1/172] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/findloc.cpp.o
278.401 [0/1/173] Linking CXX static library /home/llvm/llvm-external-buildbots/workers/ppc64-flang-aix-test/ppc64-flang-aix-build/build/lib/clang/22/lib/powerpc64-ibm-aix/libflang_rt.runtime.a
10292.561 [615/10/7764] Building CXX object lib/CAS/CMakeFiles/LLVMCAS.dir/OnDiskDataAllocator.cpp.o
FAILED: [code=1] lib/CAS/CMakeFiles/LLVMCAS.dir/OnDiskDataAllocator.cpp.o 
/home/llvm/llvm-external-buildbots/clang.20.1.7/bin/clang++ -DGTEST_HAS_RTTI=0 -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_LARGE_FILE_API -D_XOPEN_SOURCE=700 -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/llvm/llvm-external-buildbots/workers/ppc64-flang-aix-test/ppc64-flang-aix-build/build/lib/CAS -I/home/llvm/llvm-external-buildbots/workers/ppc64-flang-aix-test/ppc64-flang-aix-build/llvm-project/llvm/lib/CAS -I/home/llvm/llvm-external-buildbots/workers/ppc64-flang-aix-test/ppc64-flang-aix-build/build/include -I/home/llvm/llvm-external-buildbots/workers/ppc64-flang-aix-test/ppc64-flang-aix-build/llvm-project/llvm/include -mcmodel=large -fPIC -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=c++17  -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -pthread -MD -MT lib/CAS/CMakeFiles/LLVMCAS.dir/OnDiskDataAllocator.cpp.o -MF lib/CAS/CMakeFiles/LLVMCAS.dir/OnDiskDataAllocator.cpp.o.d -o lib/CAS/CMakeFiles/LLVMCAS.dir/OnDiskDataAllocator.cpp.o -c /home/llvm/llvm-external-buildbots/workers/ppc64-flang-aix-test/ppc64-flang-aix-build/llvm-project/llvm/lib/CAS/OnDiskDataAllocator.cpp
/home/llvm/llvm-external-buildbots/workers/ppc64-flang-aix-test/ppc64-flang-aix-build/llvm-project/llvm/lib/CAS/OnDiskDataAllocator.cpp:212:31: error: no member named 'pointer' in 'llvm::cas::OnDiskDataAllocator'
  212 | Expected<OnDiskDataAllocator::pointer>
      |          ~~~~~~~~~~~~~~~~~~~~~^
/home/llvm/llvm-external-buildbots/workers/ppc64-flang-aix-test/ppc64-flang-aix-build/llvm-project/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");
      |                            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/llvm/llvm-external-buildbots/workers/ppc64-flang-aix-test/ppc64-flang-aix-build/llvm-project/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 {
      |                                  ^~~~~~~~~
/home/llvm/llvm-external-buildbots/workers/ppc64-flang-aix-test/ppc64-flang-aix-build/llvm-project/llvm/include/llvm/CAS/OnDiskDataAllocator.h:29:7: note: OnDiskDataAllocator defined here
   29 | class OnDiskDataAllocator {
      |       ^~~~~~~~~~~~~~~~~~~
3 errors generated.
10298.631 [615/2/7772] Building CXX object utils/UnicodeData/CMakeFiles/UnicodeNameMappingGenerator.dir/UnicodeNameMappingGenerator.cpp.o
ninja: build stopped: subcommand failed.

```

</details>

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