[libc][math] Refactor expm1 implementation to header-only in src/__support/math folder. (#162127)

Part of #147386

in preparation for: https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

Author: bassiounix

libc/shared/math.h

@@ -54,6 +54,7 @@
 #include "math/exp2m1f16.h"
 #include "math/expf.h"
 #include "math/expf16.h"
+#include "math/expm1.h"
 #include "math/frexpf.h"
 #include "math/frexpf128.h"
 #include "math/frexpf16.h"

libc/shared/math/expm1.h

@@ -0,0 +1,23 @@
+//===-- Shared expm1 function -----------------------------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SHARED_MATH_EXPM1_H
+#define LLVM_LIBC_SHARED_MATH_EXPM1_H
+
+#include "shared/libc_common.h"
+#include "src/__support/math/expm1.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace shared {
+
+using math::expm1;
+
+} // namespace shared
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SHARED_MATH_EXPM1_H

libc/src/__support/math/CMakeLists.txt

@@ -871,6 +871,26 @@ add_header_library(
     libc.src.__support.macros.properties.cpu_features
 )
 
+add_header_library(
+  expm1
+  HDRS
+    expm1.h
+  DEPENDS
+    .common_constants
+    .exp_constants
+    libc.src.__support.CPP.bit
+    libc.src.__support.FPUtil.dyadic_float
+    libc.src.__support.FPUtil.fenv_impl
+    libc.src.__support.FPUtil.fp_bits
+    libc.src.__support.FPUtil.multiply_add
+    libc.src.__support.FPUtil.polyeval
+    libc.src.__support.FPUtil.rounding_mode
+    libc.src.__support.FPUtil.triple_double
+    libc.src.__support.integer_literals
+    libc.src.__support.macros.optimization
+    libc.src.errno.errno
+)
+
 add_header_library(
   range_reduction_double
   HDRS