[clang] [lld] [llvm] [mlir] [IR] Introduce `T<address space>` to `DataLayout` to represent flat address space if a target supports it (PR #108786)

[Jessica Clarke via cfe-commits]

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@@ -245,6 +246,7 @@ class DataLayout {
   unsigned getDefaultGlobalsAddressSpace() const {
     return DefaultGlobalsAddrSpace;
   }
+  unsigned getFlatAddressSpace() const { return FlatAddressSpace; }
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jrtc27 wrote:

> The absence of this specification indicates the target does not have such a flat address space to optimize away.

Yet this isn't an optional? Is the expectation that callers check against a magic ~0U sentinel? (Is there precedent for that in DataLayout?)

https://github.com/llvm/llvm-project/pull/108786