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

[via cfe-commits]

c8ef wrote:

```
Clang :: Analysis/diagnostics/sarif-diagnostics-taint-test.c
Clang :: Analysis/diagnostics/sarif-multi-diagnostic-test.c
Clang :: CXX/drs/cwg3xx.cpp
```

These test failures are unrelated and have been observed in other CI runs. Let's go ahead and merge this for now.

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