[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.
+
+The attribute generally should only be used on functions with a non-``void`` return type which do not accept a pointer or reference argument.
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AaronBallman wrote:

Hence the "generally". Inspecting the address of a pointer is a less common thing than dereferencing the pointer. I'll try to clarify in the part above though, once I figure out what to say.

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