Remove rules against std::function from the programmer's manual

Clarify that llvm::function_ref is like StringRef for callables.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213326 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ProgrammersManual.rst

@@ -298,7 +298,9 @@ The ``function_ref``
 (`doxygen <http://llvm.org/doxygen/classllvm_1_1function_ref.html>`__) class
 template represents a reference to a callable object, templated over the type
 of the callable. This is a good choice for passing a callback to a function,
-if you don't need to hold onto the callback after the function returns.
+if you don't need to hold onto the callback after the function returns. In this
+way, ``function_ref`` is to ``std::function`` as ``StringRef`` is to
+``std::string``.
 
 ``function_ref<Ret(Param1, Param2, ...)>`` can be implicitly constructed from
 any callable object that can be called with arguments of type ``Param1``,
@@ -323,17 +325,11 @@ can be called using:
       return false;
     });
 
-Note that a ``function_ref`` object contains pointers to external memory, so
-it is not generally safe to store an instance of the class (unless you know
-that the external storage will not be freed).
-``function_ref`` is small enough that it should always be passed by value.
-
-``std::function``
-^^^^^^^^^^^^^^^^^
-
-You cannot use ``std::function`` within LLVM code, because it is not supported
-by all our target toolchains.
-
+Note that a ``function_ref`` object contains pointers to external memory, so it
+is not generally safe to store an instance of the class (unless you know that
+the external storage will not be freed). If you need this ability, consider
+using ``std::function``. ``function_ref`` is small enough that it should always
+be passed by value.
 
 .. _DEBUG: