Document legacy pass manager extension points

[serge-sans-paille]

Summary

This is a follow-up to https://reviews.llvm.org/rL364937

[xbolva00]

Thank you! Looks fine.

llvm/docs/ReleaseNotes.rst

.. NOTE
    functionality, or simply have a lot to talk about), see the `NOTE` below
    for adding a new subsection.
 
 * The optimizer will now convert calls to ``memcmp`` into a calls to ``bcmp`` in
   some circumstances. Users who are building freestanding code (not depending on
   the platform's libc) without specifying ``-ffreestanding`` may need to either
   pass ``-fno-builtin-bcmp``, or provide a ``bcmp`` function.
 
+* Two new extension points, namely ``EP_FullLinkTimeOptimizationEarly`` and
+  ``EP_FullLinkTimeOptimizationLast`` are available for plugins to specialize
+  the legacy pass manager full LTO pipeline.
+
 .. NOTE
    If you would like to document a larger change, then you can add a
    subsection about it right here. You can copy the following boilerplate
    and un-indent it (the indentation causes it to be inside this comment).
 
    Special New Feature
    -------------------
 

llvm/docs/WritingAnLLVMPass.rst

 
 Lastly, we :ref:`register our class <writing-an-llvm-pass-registration>`
 ``Hello``, giving it a command line argument "``hello``", and a name "Hello
 World Pass".  The last two arguments describe its behavior: if a pass walks CFG
 without modifying it then the third argument is set to ``true``; if a pass is
 an analysis pass, for example dominator tree pass, then ``true`` is supplied as
 the fourth argument.
 
+If we want to register the pass as a step of an existing pipeline, some extension
+points are provided, e.g. ``PassManagerBuilder::EP_EarlyAsPossible`` to apply our
+pass before any optimization, or ``PassManagerBuilder::EP_FullLinkTimeOptimizationLast``
+to apply it after Link Time Optimizations.
+
+.. code-block:: c++
+
+    static llvm::RegisterStandardPasses Y(
+        llvm::PassManagerBuilder::EP_EarlyAsPossible,
+        [](const llvm::PassManagerBuilder &Builder,
+           llvm::legacy::PassManagerBase &PM) { PM.add(new Hello()); });
+
 As a whole, the ``.cpp`` file looks like:
 
 .. code-block:: c++
 
   #include "llvm/Pass.h"
   #include "llvm/IR/Function.h"
   #include "llvm/Support/raw_ostream.h"
-  
+  #include "llvm/IR/LegacyPassManager.h"
+  #include "llvm/Transforms/IPO/PassManagerBuilder.h"
+
   using namespace llvm;
-  
   namespace {
   struct Hello : public FunctionPass {
     static char ID;
     Hello() : FunctionPass(ID) {}
   
     bool runOnFunction(Function &F) override {
       errs() << "Hello: ";
       errs().write_escaped(F.getName()) << '\n';
       return false;
     }
   }; // end of struct Hello
   }  // end of anonymous namespace
-  
   char Hello::ID = 0;
   static RegisterPass<Hello> X("hello", "Hello World Pass",
                                false /* Only looks at CFG */,
                                false /* Analysis Pass */);
 
+  static RegisterStandardPasses Y(
+      PassManagerBuilder::EP_EarlyAsPossible,
+      [](const PassManagerBuilder &Builder,
+         legacy::PassManagerBase &PM) { PM.add(new Hello()); });
+
 Now that it's all together, compile the file with a simple "``gmake``" command
 from the top level of your build directory and you should get a new file
 "``lib/LLVMHello.so``".  Note that everything in this file is
 contained in an anonymous namespace --- this reflects the fact that passes
 are self contained units that do not need external interfaces (although they
 can have them) to be useful.
 
 Running a pass with ``opt``