[OpenMP] Use Clang resource dir only in bootstrapping build

[Meinersbur]

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.

Note that LLVM_ENABLE_PROJECTS=openmp is deprecated. Consider switching to a LLVM_ENABLE_RUNTIMES=openmp build and supporting removing the LLVM_ENABLE_PROJECTS=openmp build mode.

[tblah]

Thanks Michael. Makes sense to me but please wait for another review.

David and Pawel: this came up because of a missconfiguration of one of our internal CI bots. We have fixed that configuration now but I mentioned it to Michael anyway as it seemed like a regression.

[pawosm-arm]

@Meinersbur the commit message seems misleading. This is not what is deprecated:

Note that LLVM_ENABLE_RUNTIMES=openmp is deprecated.

The other thing is.
Also, s/mopde/mode/

[Meinersbur]

@Meinersbur the commit message seems misleading. This is not what is deprecated:

Thanks, this was a blunder

[Meinersbur]

I forgot that LLVM_RUNTIMES_BUILD does not imply LLVM_TREE_AVAILABLE (happens in non-boostrapping build:

llvm-project/runtimes/CMakeLists.txt

Lines 97 to 100 in 8cc1d6f

	set(LLVM_TREE_AVAILABLE OFF)
	if (LLVM_LIBRARY_OUTPUT_INTDIR AND LLVM_RUNTIME_OUTPUT_INTDIR AND PACKAGE_VERSION)
	set(LLVM_TREE_AVAILABLE ON)
	endif()

Re-added LLVM_TREE_AVAILABLE condition and switched the if/else to keep the condition simpler.

Please re-approve if you are OK with this change.