[Kaleidoscope][BuildingAJIT] Add docs for Chapter 3 of the Building A JIT

tutorial.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270917 91177308-0d34-0410-b5e6-96231b3b80d8

docs/tutorial/BuildingAJIT3.rst

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+=============================================
+Building a JIT: Per-function Lazy Compilation
+=============================================
+
+.. contents::
+   :local:
+
+**This tutorial is under active development. It is incomplete and details may
+change frequently.** Nonetheless we invite you to try it out as it stands, and
+we welcome any feedback.
+
+Chapter 3 Introduction
+======================
+
+Welcome to Chapter 3 of the "Building an ORC-based JIT in LLVM" tutorial. This
+chapter discusses lazy JITing and shows you how to enable it by adding an ORC
+CompileOnDemand layer the JIT from `Chapter 2 <BuildingAJIT2.html>`_.
+
+**To be done:**
+
+**(1) Describe lazy function-at-a-time JITing and how it differs from the kind
+of eager module-at-a-time JITing that we've been doing so far. **
+
+**(2) Discuss CompileCallbackManagers and IndirectStubManagers.**
+
+**(3) Describe CompileOnDemandLayer (automates these components and builds stubs
+and lazy compilation callbacks for IR) and how to add it to the JIT. **
+
+Full Code Listing
+=================
+
+Here is the complete code listing for our running example with a CompileOnDemand
+layer added to enable lazy function-at-a-time compilation. To build this example, use:
+
+.. code-block:: bash
+
+    # Compile
+    clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core orc native` -O3 -o toy
+    # Run
+    ./toy
+
+Here is the code:
+
+.. literalinclude:: ../../examples/Kaleidoscope/BuildingAJIT/Chapter3/KaleidoscopeJIT.h
+   :language: c++
+
+`Next: Extreme Laziness -- Using Compile Callbacks to JIT directly from ASTs <BuildingAJIT4.html>`_