docs: Add pointer to cmake caches for PGO

[thakis]

Summary

Also add a link to end-user PGO documentation.

---

The script has better SEO, but it's usually not what I want. Add a pointer for future self :)

[thakis]

end-user docs link

[george.burgess.iv]

thanks!

llvm/docs/HowToBuildWithPGO.rst

 =============================================================
 How To Build Clang and LLVM with Profile-Guided Optimizations
 =============================================================
 
 Introduction
 ============
 
 PGO (Profile-Guided Optimization) allows your compiler to better optimize code
 for how it actually runs. Users report that applying this to Clang and LLVM can
 decrease overall compile time by 20%.
 
 This guide walks you through how to build Clang with PGO, though it also applies
 to other subprojects, such as LLD.
 
 
+Using preconfigured CMake caches
+================================
+
+See https://llvm.org/docs/AdvancedBuilds.html#multi-stage-pgo
+
 Using the script
 ================
 
 We have a script at ``utils/collect_and_build_with_pgo.py``. This script is
 tested on a few Linux flavors, and requires a checkout of LLVM, Clang, and
 compiler-rt. Despite the name, it performs four clean builds of Clang, so it
 can take a while to run to completion. Please see the script's ``--help`` for
 more information on how to run it, and the different options available to you.

 have very specific needs (e.g. your compiler is meant to compile a large browser
 for four different platforms, or similar), you may want to do something else.
 This is configurable in the script itself.
 
 
 Building Clang with PGO
 =======================
 
-If you prefer to not use the script, this briefly goes over how to build
-Clang/LLVM with PGO.
+If you prefer to not use the script or the cmake cache, this briefly goes over
+how to build Clang/LLVM with PGO.
 
 First, you should have at least LLVM, Clang, and compiler-rt checked out
 locally.
 
 Next, at a high level, you're going to need to do the following:
 
 1. Build a standard Release Clang and the relevant libclang_rt.profile library
 2. Build Clang using the Clang you built above, but with instrumentation

llvm/utils/collect_and_build_with_pgo.py

 #!/usr/bin/env python3
 """
 This script:
 - Builds clang with user-defined flags
 - Uses that clang to build an instrumented clang, which can be used to collect
   PGO samples
 - Builds a user-defined set of sources (default: clang) to act as a
   "benchmark" to generate a PGO profile
 - Builds clang once more with the PGO profile generated above
 
 This is a total of four clean builds of clang (by default). This may take a
 while. :)
+
+This scripts duplicates https://llvm.org/docs/AdvancedBuilds.html#multi-stage-pgo
+Eventually, it will be updated to instead call the cmake cache mentioned there.
 """
 
 import argparse
 import collections
 import multiprocessing
 import os
 import shlex
 import shutil