[CMake] Support injecting extra dependencies for perf-training

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 "")

Differential Revision: https://reviews.llvm.org/D138974