[clang] [lld] [llvm] [DataLayout] Introduce sentinel pointer value (PR #131557)

[Alexander Richardson via cfe-commits]

arichardson wrote:

Since #105735 we now have `p<flags>` in the DataLayout syntax. We could allow the `z` character for sentinel=zeroes (the default) and `o` for sentinel=ones? This would simplify the parsing and avoid adding another component unless anyone thinks we will eventually support arbitrary bit patterns?

https://github.com/llvm/llvm-project/pull/131557