[clang] [Clang][perf-training] Do build of libLLVMSupport for perf training (PR #111625)

[Tom Stellard via cfe-commits]

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+RUN: %cmake -G %cmake_generator -B %t -S %llvm_src_dir -DCMAKE_C_COMPILER=%clang -DCMAKE_CXX_COMPILER=%clang -DCMAKE_CXX_FLAGS="--driver-mode=g++" -DCMAKE_BUILD_TYPE=Release
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tstellar wrote:

The current bolt.lit.cfg substitutes `%clang_cpp` to `clang --driver-mode=g++` because there is no clang++ symlink to the instrumented bolt binary.  So this option is required in order to use `clang` as a c++ compiler.  The other alternative solution would be to add the clang++ symlink.

https://github.com/llvm/llvm-project/pull/111625