[clang] [clang] constexpr built-in abs function. (PR #112539)

[Timm Baeder via cfe-commits]

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@@ -265,6 +265,15 @@ namespace fpclassify {
   char classify_subnorm [__builtin_fpclassify(-1, -1, -1, +1, -1, 1.0e-38f)];
 }
 
+namespace abs {
+static_assert(__builtin_abs(14) == 14, "");
+static_assert(__builtin_labs(14L) == 14L, "");
+static_assert(__builtin_llabs(14LL) == 14LL, "");
+static_assert(__builtin_abs(-14) == 14, "");
+static_assert(__builtin_labs(-0x14L) == 0x14L, "");
+static_assert(__builtin_llabs(-0x141414141414LL) == 0x141414141414LL, "");
----------------
tbaederr wrote:

```suggestion
  static_assert(__builtin_abs(14) == 14, "");
  static_assert(__builtin_labs(14L) == 14L, "");
  static_assert(__builtin_llabs(14LL) == 14LL, "");
  static_assert(__builtin_abs(-14) == 14, "");
  static_assert(__builtin_labs(-0x14L) == 0x14L, "");
  static_assert(__builtin_llabs(-0x141414141414LL) == 0x141414141414LL, "");
```

https://github.com/llvm/llvm-project/pull/112539