When doing a multi-stage PGO build of clang, run the check-clang and
check-llvm targets using the instrumented clang and use that profile
data for building the final stage2 clang.  This is what is recommended
by our official documentation: https://llvm.org/docs/HowToBuildWithPGO.html#building-clang-with-pgo

I benchmarked this change by compiling the SemaChecking.cpp file from
clang.  Using check-clang/check-llvm to generate the profile data gives a 25% speedup
in the PGO+LTO stage2 clang when compared to the stage1 clang (no-LTO).

Prior to this change, I was only seeing ~5% speedup when comparing the
stage2 and stage1 builds.

check-llvm

Also add an option to supply a cmake project to use to generate profile
data.