[libc++] Avoid potential truncation warnings in std::abs test

[ldionne]

Summary

One some platforms, -Wimplicit-int-conversion is enabled by default,
which can lead to additional warnings being triggered in this test.
Since we're only trying to test errors related to calling abs(), the
assignment is superfluous.

As a fly-by fix, correct one instance of ::abs to std::abs.

[Mordante]

LGTM. Interesting to see different diagnostics.

[jloser]

LGTM. You should be able to remove #include "test_macros.h" if you feel like it.

[ldionne]

In D114244#3142822, @jloser wrote:

LGTM. You should be able to remove #include "test_macros.h" if you feel like it.

Removed upon committing.

libcxx/test/std/numerics/c.math/abs.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include <cmath>
 
-#include "test_macros.h"
-
-int main(int, char**)
-{
+void f() {
     unsigned int ui = -5;
-    ui = std::abs(ui); // expected-error {{call to 'abs' is ambiguous}}
+    (void)std::abs(ui); // expected-error {{call to 'abs' is ambiguous}}
 
     unsigned char uc = -5;
-    uc = std::abs(uc); // expected-warning {{taking the absolute value of unsigned type 'unsigned char' has no effect}}
+    (void)std::abs(uc); // expected-warning {{taking the absolute value of unsigned type 'unsigned char' has no effect}}
 
     unsigned short us = -5;
-    us = std::abs(us); // expected-warning {{taking the absolute value of unsigned type 'unsigned short' has no effect}}
+    (void)std::abs(us); // expected-warning {{taking the absolute value of unsigned type 'unsigned short' has no effect}}
 
     unsigned long ul = -5;
-    ul = std::abs(ul); // expected-error {{call to 'abs' is ambiguous}}
+    (void)std::abs(ul); // expected-error {{call to 'abs' is ambiguous}}
 
     unsigned long long ull = -5;
-    ull = ::abs(ull); // expected-error {{call to 'abs' is ambiguous}}
-
-    return 0;
+    (void)std::abs(ull); // expected-error {{call to 'abs' is ambiguous}}
 }