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

This reverts commit r292883. Unfortunately <string_view> uses
_LIBCPP_ASSERT in a way which is not compatible with the C++11 dylib
build. I'll investigate more tomorrow.

llvm-svn: 292923
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: f9e7bf3a43fd28c9ffd1523f1c832275e80a9f85
diff --git a/src/mutex.cpp b/src/mutex.cpp
index b858e88..338b79f 100644
--- a/src/mutex.cpp
+++ b/src/mutex.cpp
@@ -11,6 +11,7 @@
 #include "mutex"
 #include "limits"
 #include "system_error"
+#include "cassert"
 #include "include/atomic_support.h"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
@@ -44,7 +45,7 @@
 {
     int ec = __libcpp_mutex_unlock(&__m_);
     (void)ec;
-    _LIBCPP_ASSERT(ec == 0, "call to mutex::unlock failed");
+    assert(ec == 0);
 }
 
 // recursive_mutex
@@ -60,7 +61,7 @@
 {
     int e = __libcpp_recursive_mutex_destroy(&__m_);
     (void)e;
-    _LIBCPP_ASSERT(e == 0, "call to ~recursive_mutex() failed");
+    assert(e == 0);
 }
 
 void
@@ -76,7 +77,7 @@
 {
     int e = __libcpp_recursive_mutex_unlock(&__m_);
     (void)e;
-    _LIBCPP_ASSERT(e == 0, "call to recursive_mutex::unlock() failed");
+    assert(e == 0);
 }
 
 bool