[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.

Author: Meinersbur

openmp/runtime/src/CMakeLists.txt

@@ -10,11 +10,12 @@
 
 include(ExtendPath)
 
-# The generated headers will be placed in clang's resource directory if present.
-if(NOT LLVM_TREE_AVAILABLE)
-  set(LIBOMP_HEADERS_INTDIR ${CMAKE_CURRENT_BINARY_DIR})
-else()
+# The generated headers will be placed in clang's resource if using it to
+# compile openmp in a runtimes-bootstrapping build.
+if(LLVM_TREE_AVAILABLE AND LLVM_RUNTIMES_BUILD)
   set(LIBOMP_HEADERS_INTDIR ${LLVM_BINARY_DIR}/${LIBOMP_HEADERS_INSTALL_PATH})
+else()
+  set(LIBOMP_HEADERS_INTDIR ${CMAKE_CURRENT_BINARY_DIR})
 endif()
 
 # Configure omp.h, kmp_config.h and omp-tools.h if necessary