[clang][perf-training] Support excluding LLVM build from PGO training (#126876)

Using LLVM build itself for PGO training is convenient and a great
starting point but it also has several issues:

* LLVM build implicitly depends on tools other than CMake and C/C++
compiler and if those tools aren't available in PATH, the build will
fail.
* LLVM build also requires standard headers and libraries which may not
always be available in the default location requiring an explicit
sysroot.
* Building a single configuration (-DCMAKE_BUILD_TYPE=Release) only
exercises the -O3 pipeline and can pesimize other configurations.
* Building for the host target doesn't exercise all other targets.
* Since LLVMSupport is a static library, this doesn't exercise the
linker (beyond what the CMake itself does).

Rather than using LLVM build, ideally we would provide a more minimal,
purpose built corpus. While we're working on building such a corpus,
provide a CMake option that lets vendors disable the use LLVM build for
PGO training.

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