[libcxx] guard throw with exception enabling check

Summary: this fixes build error when built with c++14 and no exceptions

Reviewers: rmaprath

Subscribers: weimingz, grandinj, rmaprath, cfe-commits

Differential Revision: http://reviews.llvm.org/D21673

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@273697 91177308-0d34-0410-b5e6-96231b3b80d8

include/experimental/optional

@@ -517,7 +517,11 @@ public:
     constexpr value_type const& value() const
     {
         if (!this->__engaged_)
+#ifndef _LIBCPP_NO_EXCEPTIONS
             throw bad_optional_access();
+#else
+            assert(!"bad optional access");
+#endif
         return this->__val_;
     }
 
@@ -525,7 +529,11 @@ public:
     value_type& value()
     {
         if (!this->__engaged_)
+#ifndef _LIBCPP_NO_EXCEPTIONS
             throw bad_optional_access();
+#else
+            assert(!"bad optional access");
+#endif
         return this->__val_;
     }