[clang] [clang][CodeGen] Add query for a target's flat address space (PR #95728)

[Matt Arsenault via cfe-commits]

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@@ -1764,6 +1764,13 @@ class TargetInfo : public TransferrableTargetInfo,
     return 0;
   }
 
+  /// \returns Target specific flat ptr address space; a flat ptr is a ptr that
+  /// can be casted to / from all other target address spaces. If the target
+  /// exposes no such address space / does not care, we return 0.
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arsenm wrote:

This further spreads the notion that 0 is a lack of address space, which it is not. You shouldn't need a new thing for this; it should be equivalent to querying the target address space map for the LangAS Generic thing 

https://github.com/llvm/llvm-project/pull/95728