[CMake][PGO] Add option for using an external project to generate profile data (#78879)

The new CLANG_PGO_TRAINING_DATA_SOURCE_DIR allows users to specify a
CMake project to use for generating the profile data. For example, to
use the llvm-test-suite to generate profile data you would do:

$ cmake -G Ninja -B build -S llvm -C <path to
source>/clang/cmake/caches/PGO.cmake \
-DBOOTSTRAP_CLANG_PGO_TRAINING_DATA_SOURCE_DIR=<path to llvm-test-suite>
\
        -DBOOTSTRAP_CLANG_PGO_TRAINING_DEPS=runtimes

Note that the CLANG_PERF_TRAINING_DEPS has been renamed to
CLANG_PGO_TRAINING_DEPS.

---------

Co-authored-by: Petr Hosek <phosek@google.com>