[OpenMP] Use Clang resource dir only in bootstrapping build (#156018) In an LLVM_ENABLE_PROJECTS=openmp build, the LLVM build tree in which just-built Clang is available, but in contrast to an LLVM_ENABLE_RUNTIMES=openmp build, is not the compiler that openmp is built with (CMAKE_CXX_COMPILER). The latter compiler (which might also be gcc) will not look into the resource directory of just-built Clang, where the OpenMP headers are installed. There may not even be a just-built Clang without LLVM_ENABLE_PROJECTS=clang. We cannot add the OpenMP header output directory to the search path which also include's Clang's internal headers that will conflict with CMAKE_CXX_COMPILER's internal headers. The only choice left is to use what the OpenMP standalone build does: Use CMAKE_CURRENT_BINARY_DIR which is added unconditionally to the header search path to compile openmp itself.
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