[clang] [clang][Interp] Implement __builtin_popcount() (PR #67929)

[Erich Keane via cfe-commits]

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@@ -260,3 +260,12 @@ namespace SourceLocation {
     static_assert(c.a.n == __LINE__ - 1, "");
   }
 }
+
+namespace popcount {
+  static_assert(__builtin_popcount(~0u) == __CHAR_BIT__ * sizeof(unsigned int), "");
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erichkeane wrote:

I would suggest more 'difficult' tests here, something other than fully saturated. 

https://github.com/llvm/llvm-project/pull/67929