[clang] [clang] Add the candiscard attribute to suppress nodiscard (PR #154943)

[Nikolas Klauser via cfe-commits]

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@@ -65,15 +65,31 @@ void GH104391() {
   M; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
 }
 
+struct S4 get_s_ignored(void) __attribute__((candiscard));
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philnik777 wrote:

There doesn't seem to be any testing for what happens if you add both `candiscard` and `nodiscard` to a function.

https://github.com/llvm/llvm-project/pull/154943