Extracted uncaught_exceptions function from Boost.Log.

The uncaught_exceptions function is functionally equivalent to
unhandled_exceptions_count in Boost.Log and implements functionality
similar to the same named C++17 standard function. Tests and docs are
also included.

One notable difference from std::uncaught_exceptions is that the return
type is unsigned rather than signed. This is deliberate as uncaught_exceptions
must never return a negative value and unsigned int better documents that.
Theoretically, as a counter, it may also overflow.

doc/core.qbk

@@ -57,3 +57,4 @@ criteria for inclusion is that the utility component be:
 [include scoped_enum.qbk]
 [include swap.qbk]
 [include typeinfo.qbk]
+[include uncaught_exceptions.qbk]

doc/uncaught_exceptions.qbk

@@ -0,0 +1,52 @@
+[/
+ / Copyright (c) 2018 Andrey Semashev
+ /
+ / Distributed under the Boost Software License, Version 1.0. (See accompanying
+ / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ /]
+
+[section:uncaught_exceptions uncaught_exceptions]
+
+[simplesect Authors]
+
+* Andrey Semashev
+
+[endsimplesect]
+
+[section Header <boost/core/uncaught_exceptions.hpp>]
+
+The header `<boost/core/uncaught_exceptions.hpp>` defines the `boost::core::uncaught_exceptions` function,
+which is a more portable implementation of the same named function introduced in C++17. The function
+returns the number of the currently pending exceptions. When that function returns a value greater than 0,
+throwing an exception from a destructor can terminate the program.
+
+Unfortunately, the function cannot be implemented on every pre-C++17 compiler, although the most commonly
+used compilers are supported. When the compiler does not provide the necessary functionality,
+`boost::core::uncaught_exceptions` returns a non-zero value if at least one exception is pending (i.e. not
+necessarily the number of pending exceptions), and `BOOST_CORE_UNCAUGHT_EXCEPTIONS_EMULATED` macro
+is defined.
+
+[section Example]
+``
+class my_class
+{
+private:
+    const unsigned int m_exception_count;
+
+public:
+    my_class() : m_exception_count(boost::core::uncaught_exceptions())
+    {
+    }
+
+    ~my_class() noexcept(false)
+    {
+        if (m_exception_count == boost::core::uncaught_exceptions())
+            do_something_potentially_throwing();
+    }
+};
+``
+[endsect]
+
+[endsect]
+
+[endsect]