[libc] Support LIBC_COPT_USE_C_ASSERT build flag

[mcgrathr]

Summary

In this mode, LIBC_ASSERT is just standard C assert.

libc/src/__support/libc_assert.h

 //===-- Definition of a libc internal assert macro --------------*- 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_SRC_SUPPORT_LIBC_ASSERT_H
 #define LLVM_LIBC_SRC_SUPPORT_LIBC_ASSERT_H
 
+#ifdef LIBC_COPT_USE_C_ASSERT
+
+// The build is configured to just use the public <assert.h> API
+// for libc's internal assertions.
+
+#include <assert.h>
+
+#define LIBC_ASSERT(COND) assert(COND)
+
+#else // Not LIBC_COPT_USE_C_ASSERT
+
 #include "src/__support/OSUtil/io.h"
 #include "src/__support/OSUtil/quick_exit.h"
 #include "src/__support/integer_to_string.h"
 #include "src/__support/macros/attributes.h" // For LIBC_INLINE
 
 namespace __llvm_libc {
 
 LIBC_INLINE void report_assertion_failure(const char *assertion,
do {                                                                         \
     if (!(COND)) {                                                             \
       __llvm_libc::report_assertion_failure(#COND, __FILE__, __LINE__,         \
                                             __PRETTY_FUNCTION__);              \
       __llvm_libc::quick_exit(0xFF);                                           \
     }                                                                          \
   } while (false)
 #endif // NDEBUG
 
+#endif // LIBC_COPT_USE_C_ASSERT
+
 #endif // LLVM_LIBC_SRC_SUPPORT_LIBC_ASSERT_H