Provide std::abs(<floating-point>) in <cmath> on Solaris.

1) <cstdlib> header should define std::abs([int|long|long long])
functions. They use "using ::abs" to import these functions (which are
declared in <stdlib.h>) into std namespace.
2) <cmath> header should define std::abs([float|double|long double])
function. If we try define new functions in std namespace, then it
will cause compile error in <cstdlib> because "using ::abs" will try
import not only [int|long|long long] functions, but also
[float|double|long double] which are defined in <math.h> header on
solaris.

Patch by C Bergstrom.

llvm-svn: 232641
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: b2c276e4480d9a26b679a59321bdd8fd449e97db
diff --git a/include/cmath b/include/cmath
index b340b4d..5f9aaed 100644
--- a/include/cmath
+++ b/include/cmath
@@ -654,6 +654,10 @@
 
 // abs
 
+#if defined(__sun__)
+using ::abs;
+#endif
+
 #if !defined(_AIX) && !defined(__sun__)
 inline _LIBCPP_INLINE_VISIBILITY
 float