[UsersManual] Add llvm-progen as an alternative tool for AutoFDO profile generation.

[hoy]

Summary

I'm adding llvm-profgen as an alternative AutoFDO profile generator to the user manual. llvm-profgen is widely used and tested by META as their default profile generator.

[wenlei]

Given llvm-profgen is in llvm tree, I'm wondering if it makes sense to treat it first option instead of alternative to out of tree create_llvm_prof. wdyt @davidxl

[hoy]

In D142994#4094193, @wenlei wrote:

Given llvm-profgen is in llvm tree, I'm wondering if it makes sense to treat it first option instead of alternative to out of tree create_llvm_prof. wdyt @davidxl

@davidxl any thought on this? Thanks.

[davidxl]

lgtm

clang/docs/UsersManual.rst

.. code-block:: console
 
      $ create_llvm_prof --binary=./code --out=code.prof
 
    This will read ``perf.data`` and the binary file ``./code`` and emit
    the profile data in ``code.prof``. Note that if you ran ``perf``
    without the ``-b`` flag, you need to use ``--use_lbr=false`` when
    calling ``create_llvm_prof``.
 
+   Alternatively, the LLVM tool ``llvm-profgen`` can also be used to generate
+   the LLVM sample profile:
+
+   .. code-block:: console
+
+     $ llvm-profgen --binary=./code --output=code.prof--perfdata=perf.data
+
+
 4. Build the code again using the collected profile. This step feeds
    the profile back to the optimizers. This should result in a binary
    that executes faster than the original one. Note that you are not
    required to build the code with the exact same arguments that you
    used in the first step. The only requirement is that you build the code
    with ``-gline-tables-only`` and ``-fprofile-sample-use``.
 
    .. code-block:: console