[CMake] Support injecting extra dependencies for perf-training

[phosek]

Summary

It may be necessary to build additional targets before running
perf-training, the typical use case would be builtins and runtimes.

This change allows users to specify those dependencies as:

set(CLANG_PERF_TRAINING_DEPS builtins runtimes CACHE STRING "")

[smeenai]

Might be worth a mention in either https://llvm.org/docs/AdvancedBuilds.html#multi-stage-pgo, to make it a bit more discoverable.

clang/utils/perf-training/CMakeLists.txt

 set(CLANG_PGO_TRAINING_DATA "${CMAKE_CURRENT_SOURCE_DIR}" CACHE PATH
   "The path to a lit testsuite containing samples for PGO and order file generation"
   )
 
 if(LLVM_BUILD_INSTRUMENTED)
   configure_lit_site_cfg(
     ${CMAKE_CURRENT_SOURCE_DIR}/lit.site.cfg.in
     ${CMAKE_CURRENT_BINARY_DIR}/pgo-data/lit.site.cfg
     )
 
   add_lit_testsuite(generate-profraw "Generating clang PGO data"
     ${CMAKE_CURRENT_BINARY_DIR}/pgo-data/
     EXCLUDE_FROM_CHECK_ALL
-    DEPENDS clang clear-profraw
+    DEPENDS clang clear-profraw ${CLANG_PERF_TRAINING_DEPS}
     )
 
   add_custom_target(clear-profraw
     COMMAND "${Python3_EXECUTABLE}" ${CMAKE_CURRENT_SOURCE_DIR}/perf-helper.py clean ${CMAKE_CURRENT_BINARY_DIR} profraw
     COMMENT "Clearing old profraw data")
 
   if(NOT LLVM_PROFDATA)
     find_program(LLVM_PROFDATA llvm-profdata)

llvm/docs/AdvancedBuilds.rst

 
 .. code-block:: console
 
   <build dir>/tools/clang/stage2-instrumented-bins/utils/perf-training/clang.profdata
 
 You can feed that file into the LLVM_PROFDATA_FILE option when you build your
 optimized compiler.
 
+It may be necessary to build additional targets before running perf training, such as
+builtins and runtime libraries. You can use the :code:`CLANG_PERF_TRAINING_DEPS` CMake
+variable for that purpose:
+
+.. code-block:: cmake
+
+  set(CLANG_PERF_TRAINING_DEPS builtins runtimes CACHE STRING "")
+
 The PGO cache has a slightly different stage naming scheme than other
 multi-stage builds. It generates three stages: stage1, stage2-instrumented, and
 stage2. Both of the stage2 builds are built using the stage1 compiler.
 
 The PGO cache generates the following additional targets:
 
 **stage2-instrumented**
   Builds a stage1 compiler, runtime, and required tools (llvm-config,