[libcxx] Never use <cassert> within libc++

Summary:
It is my opinion that libc++ should never use `<cassert>`, including in the `dylib`. This patch remove all uses of `assert` from within libc++ and replaces most of them with `_LIBCPP_ASSERT` instead.

Additionally this patch turn `LIBCXX_ENABLE_ASSERTIONS`  off by default, because the standard library should not be aborting user programs unless explicitly asked to.

Reviewers: mclow.lists, compnerd, smeenai

Reviewed By: mclow.lists

Subscribers: mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D29063

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@292883 91177308-0d34-0410-b5e6-96231b3b80d8

src/system_error.cpp

@@ -17,9 +17,9 @@
 #include "cstring"
 #include "cstdio"
 #include "cstdlib"
-#include "cassert"
 #include "string"
 #include "string.h"
+#include "__debug"
 
 #if defined(__ANDROID__)
 #include <android/api-level.h>
@@ -96,7 +96,7 @@ string do_strerror_r(int ev) {
             std::snprintf(buffer, strerror_buff_size, "Unknown error %d", ev);
             return string(buffer);
         } else {
-            assert(new_errno == ERANGE);
+            _LIBCPP_ASSERT(new_errno == ERANGE, "unexpected error from ::strerr_r");
             // FIXME maybe? 'strerror_buff_size' is likely to exceed the
             // maximum error size so ERANGE shouldn't be returned.
             std::abort();