[clang] Document the const and pure attributes (PR #205881)

[Aaron Ballman via cfe-commits]

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@@ -10253,3 +10253,52 @@ The attribute is also supported with blocks and in Objective-C.
   }
   }];
 }
+
+def ConstDocs :  Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+The ``const`` attribute can be applied to the declaration of a function to signal that repeated calls to the function with the same argument values may be safe to elide because the subsequent calls will always return the same value as the initial call.
+
+The attribute informs the optimizer that the function cannot read or write to memory, does not support unwinding, will return (has no infinite loops), and that any pointer or reference arguments to the call will not be read from or written to.
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AaronBallman wrote:

I documented #42620 because I don't believe that behavior is going to change.

But do we consider #13130 a bug? My impression is that we intend to fix that behavior at some point but it's an ancient bug so I'm trying to figure out whether we're basically stuck with what we've got.

As for #36098 I'm not certain if I've addressed your concerns or not; I reworded a bit more with one of the latest changes. If there's more here you'd like to have said, let me know.

https://github.com/llvm/llvm-project/pull/205881