[Docs] Add Documentation on BOLT Build Configs

This patch adds documentation into the advanced builds documentation on
how to use the BOLT caches, including the combinations with the PGO
multistage builds and (Thin)LTO.

Reviewed By: sylvestre.ledru, Amir

Differential Revision: https://reviews.llvm.org/D137899

llvm/docs/AdvancedBuilds.rst

@@ -195,6 +195,53 @@ The PGO cache generates the following additional targets:
   Depends on stage2 and runs the test-suite using the stage2 compiler (requires
   in-tree test-suite).
 
+BOLT
+====
+
+`BOLT <https://github.com/llvm/llvm-project/blob/main/bolt/README.md>`_
+(Binary Optimization and Layout Tool) is a tool that optimizes binaries
+post-link by profiling them at runtime and then using that information to
+optimize the layout of the final binary among other optimizations performed
+at the binary level. There are also CMake caches available to build
+LLVM/Clang with BOLT.
+
+To configure a single-stage build that builds LLVM/Clang and then optimizes
+it with BOLT, use the following CMake configuration:
+
+.. code-block:: console
+
+  $ cmake <path to source>/llvm -C <path to source>/clang/cmake/caches/BOLT.cmake
+
+Then, build the BOLT-optimized binary by running the following ninja command:
+
+.. code-block:: console
+
+  $ ninja clang++-bolt
+
+If you're seeing errors in the build process, try building with a recent
+version of Clang/LLVM by setting the CMAKE_C_COMPILER and
+CMAKE_CXX_COMPILER flags to the appropriate values.
+
+It is also possible to use BOLT on top of PGO and (Thin)LTO for an even more
+significant runtime speedup. To configure a three stage PGO build with ThinLTO
+that optimizes the resulting binary with BOLT, use the following CMake
+configuration command:
+
+.. code-block:: console
+
+  $ cmake -G Ninja <path to source>/llvm \
+      -C <path to source>/clang/cmake/caches/BOLT-PGO.cmake \
+      -DBOOTSTRAP_LLVM_ENABLE_LLD=ON \
+      -DBOOTSTRAP_BOOTSTRAP_LLVM_ENABLE_LLD=ON \
+      -DPGO_INSTRUMENT_LTO=Thin
+
+Then, to build the final optimized binary, build the stage2-clang++-bolt
+target:
+
+.. code-block:: console
+
+  $ ninja stage2-clang++-bolt
+
 3-Stage Non-Determinism
 =======================