starting from llvm14 the install prefix breaks via symlinks;
/usr/lib/llvm14/lib/cmake/llvm/LLVMConfig.cmake goes up 3 directories to find
/usr/lib/llvm14/include as LLVM_INCLUDE_DIRS, but to even use this cmake folder
at all it has to be symlinked to /usr/lib/cmake/llvm .. so the directory it
instead uses is just /usr/include, which is not where the cmake includes are.
this hardcodes them to the install prefix we pass via cmake, which should
always be correct, and what cmake tries to autodetect anyway.

also see: https://reviews.llvm.org/D29969

this is supposedly fixed now, but for some reason it still isn't
--- a/llvm/cmake/modules/CMakeLists.txt
+++ b/llvm/cmake/modules/CMakeLists.txt
@@ -41,6 +41,8 @@
 #
 
 set(LLVM_CONFIG_CODE "
+# this is wrong when automatically detected
+set(LLVM_INSTALL_PREFIX \"${CMAKE_INSTALL_PREFIX}\")
 # LLVM_BUILD_* values available only from LLVM build tree.
 set(LLVM_BUILD_BINARY_DIR \"${LLVM_BINARY_DIR}\")
 set(LLVM_BUILD_LIBRARY_DIR \"${LLVM_LIBRARY_DIR}\")
@@ -109,8 +111,6 @@
 #
 # Generate LLVMConfig.cmake for the install tree.
 #
-
-find_prefix_from_config(LLVM_CONFIG_CODE LLVM_INSTALL_PREFIX "${LLVM_INSTALL_PACKAGE_DIR}")
 
 extend_path(LLVM_CONFIG_MAIN_INCLUDE_DIR "\${LLVM_INSTALL_PREFIX}" "${CMAKE_INSTALL_INCLUDEDIR}")
 # This is the same as the above because the handwritten and generated headers