[clang-repl] Add basic documentation about clang-repl

[v.g.vassilev]

This looks good to me. I can commit that change for you as I know you do not have commit access.

[v.g.vassilev]

Sorry, selected the wrong option. This looks good to me. I can commit that change for you as I know you do not have commit access.

[v.g.vassilev]

I have modified it a little before landing it to capture better the intent and our off-list conversation.

clang/docs/ClangRepl.rst

+===========
+Clang-Repl
+===========
+
+**Clang-Repl** is an interactive C++ interpreter that allows for incremental
+compilation. It supports interactive programming for C++ in a
+read-evaluate-print-loop (REPL) style. It uses Clang as a library to compile the
+high level programming language into LLVM IR. Then the LLVM IR is executed by
+the LLVM just-in-time (JIT) infrastructure.
+
+Clang-Repl is suitable for exploratory programming and in places where time
+to insight is important. Clang-Repl is a project inspired by the work in
+`Cling <https://github.com/root-project/cling>`_, a LLVM-based C/C++ interpreter
+developed by the field of high energy physics and used by the scientific data
+analysis framework `ROOT <https://root.cern/>`_. Clang-Repl allows to move parts
+of Cling upstream, making them useful and available to a broader audience.
+
+
+
+Clang-Repl Usage
+================
+
+
+.. code-block:: c++
+
+  clang-repl> #include <iostream>
+  clang-repl> int f() { std::cout << "Hello Interpreted World!\n"; return 0; }
+  clang-repl> auto r = f();
+  // Prints Hello Interpreted World!
+
+Note that the implementation is not complete and highly experimental. We do
+not yet support statements on the global scope, for example.
+
+
+Clang-Repl Basic Data Flow
+==========================
+
+.. image:: ClangRepl_design.png
+   :align: center
+   :alt: ClangRepl design
+
+Clang-Repl data flow can be divided into roughly 8 phases:
+
+1. Clang-Repl controls the input infrastructure by an interactive prompt or by
+   an interface allowing the incremental processing of input.
+
+2. Then it sends the input to the underlying incremental facilities in Clang
+   infrastructure.
+
+3. Clang compiles the input into an AST representation.
+
+4. When required the AST can be further transformed in order to attach specific
+   behavior.
+
+5. The AST representation is then lowered to LLVM IR.
+
+6. The LLVM IR is the input format for LLVM’s JIT compilation infrastructure.
+   The tool will instruct the JIT to run specified functions, translating them
+   into machine code targeting the underlying device architecture (eg. Intel
+   x86 or NVPTX).
+
+7. The LLVM JIT lowers the LLVM IR to machine code.
+
+8. The machine code is then executed.
+
+
+Just like Clang, Clang-Repl can be integrated in existing applications as a
+library (via using the clangInterpreter library). This turning your C++ compiler
+into a service which incrementally can consume and execute code. The
+**Compiler as A Service** (**CaaS**) concept helps supporting move advanced use
+cases such as template instantiations on demand and automatic language
+interoperability. It also helps static languages such as C/C++ become apt for
+data science.
+
+
+Related Reading
+===============
+`Cling Transitions to LLVM's Clang-Repl <https://root.cern/blog/cling-in-llvm/>`_
+
+`Moving (parts of) the Cling REPL in Clang <https://lists.llvm.org/pipermail/llvm-dev/2020-July/143257.html>`_
+
+`GPU Accelerated Automatic Differentiation With Clad <https://arxiv.org/pdf/2203.06139.pdf>`_

clang/docs/index.rst

.. toctree::
    ClangFormat
    ClangFormatStyleOptions
    ClangFormattedStatus
    ClangLinkerWrapper
    ClangNvlinkWrapper
    ClangOffloadBundler
    ClangOffloadWrapper
    ClangOffloadPackager
+   ClangRepl
 
 Design Documents
 ================
 
 .. toctree::
    :maxdepth: 1
 
    CodeOwners