[llvm] [CAS] Add llvm-cas tools to inspect on-disk LLVMCAS (PR #166481)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-armv7-global-isel` running on `linaro-clang-armv7-global-isel` while building `llvm` at step 7 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/39/builds/8594

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

```
Step 7 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'LLVM :: tools/llvm-cas/dump.test' FAILED ********************
Exit Code: -6

Command Output (stdout):
--
# RUN: at line 1
rm -rf /home/tcwg-buildbot/worker/clang-armv7-global-isel/stage1/test/tools/llvm-cas/Output/dump.test.tmp
# executed command: rm -rf /home/tcwg-buildbot/worker/clang-armv7-global-isel/stage1/test/tools/llvm-cas/Output/dump.test.tmp
# RUN: at line 2
mkdir /home/tcwg-buildbot/worker/clang-armv7-global-isel/stage1/test/tools/llvm-cas/Output/dump.test.tmp
# executed command: mkdir /home/tcwg-buildbot/worker/clang-armv7-global-isel/stage1/test/tools/llvm-cas/Output/dump.test.tmp
# RUN: at line 4
/home/tcwg-buildbot/worker/clang-armv7-global-isel/stage1/bin/llvm-cas --cas /home/tcwg-buildbot/worker/clang-armv7-global-isel/stage1/test/tools/llvm-cas/Output/dump.test.tmp/cas --make-blob    --data - </dev/null
# executed command: /home/tcwg-buildbot/worker/clang-armv7-global-isel/stage1/bin/llvm-cas --cas /home/tcwg-buildbot/worker/clang-armv7-global-isel/stage1/test/tools/llvm-cas/Output/dump.test.tmp/cas --make-blob --data -
# .---command stderr------------
# | llvm-cas: ../llvm/llvm/lib/Support/Unix/Path.inc:837: std::error_code llvm::sys::fs::mapped_file_region::init(int, uint64_t, mapmode): Assertion `Size != 0' failed.
# | PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and instructions to reproduce the bug.
# | Stack dump:
# | 0.	Program arguments: /home/tcwg-buildbot/worker/clang-armv7-global-isel/stage1/bin/llvm-cas --cas /home/tcwg-buildbot/worker/clang-armv7-global-isel/stage1/test/tools/llvm-cas/Output/dump.test.tmp/cas --make-blob --data -
# | #0 0x0e5c38a0 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/tcwg-buildbot/worker/clang-armv7-global-isel/stage1/bin/llvm-cas+0x338a0)
# | #1 0x0e5c11bc llvm::sys::RunSignalHandlers() (/home/tcwg-buildbot/worker/clang-armv7-global-isel/stage1/bin/llvm-cas+0x311bc)
# | #2 0x0e5c4bc8 SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
# | #3 0xf5ccd6f0 __default_rt_sa_restorer ./signal/../sysdeps/unix/sysv/linux/arm/sigrestorer.S:80:0
# | #4 0xf5cbdb06 ./csu/../sysdeps/unix/sysv/linux/arm/libc-do-syscall.S:47:0
# | #5 0xf5cfd292 __pthread_kill_implementation ./nptl/pthread_kill.c:44:76
# | #6 0xf5ccc840 gsignal ./signal/../sysdeps/posix/raise.c:27:6
# `-----------------------------
# error: command failed with exit status: -6

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/166481