[clang] [llvm] [ADT] Add and use (for AArch64) `ValueOrSentinel<T, Sentinel>` (PR #158120)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

Note: Failing tests are:
```
FAIL: lldb-api::TestVectorOfVectorsFromStdModule.py
FAIL: lldb-api::TestDbgInfoContentVectorFromStdModule.py
```
Which appear unrelated (and failing on other PRs too).

https://github.com/llvm/llvm-project/pull/158120