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

[Matt Arsenault via llvm-commits]

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@@ -552,6 +553,11 @@ class DataLayout {
   ///
   /// This includes an explicitly requested alignment (if the global has one).
   Align getPreferredAlign(const GlobalVariable *GV) const;
+
+  /// Returns the sentinel pointer value for a given address space. If the
+  /// address space is invalid, it defaults to the sentinel pointer value of
+  /// address space 0, aligning with the behavior of \p getPointerSpec.
+  APInt getSentinelPointerValue(unsigned AS) const;
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arsenm wrote:

If you aren't going to allow an arbitrary bit pattern for this, and only the 2 special cases, I'd rather this just return an enum over those values than the heavier APInt 

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