Cherry pick CL2012695

Switch llvm to llvm-project

Change-Id: Ib815e98f76bd413af5bd2ffd47058241cda319fc

CMakeLists.txt

@@ -147,7 +147,13 @@ set(PAL_BUILD_VEGA20 ${XGL_BUILD_VEGA20} CACHE BOOL "${PROJECT_NAME} override."
 set(PAL_BUILD_GFX10 ${XGL_BUILD_GFX10} CACHE BOOL "${PROJECT_NAME} override." FORCE)
 
 # LLVM
-set(XGL_LLVM_SRC_PATH ${PROJECT_SOURCE_DIR}/../llvm CACHE PATH "Specify the path to the LLVM.")
+# LLVM will be switched to llvm monorepo. Check if the new llvm exist first and fallback to
+# the old llvm if it doesn't during the transition. The old path should be removed once the transition gets completed.
+if(EXISTS ${PROJECT_SOURCE_DIR}/../llvm-project/llvm)
+    set(XGL_LLVM_SRC_PATH ${PROJECT_SOURCE_DIR}/../llvm-project/llvm CACHE PATH "Specify the path to the LLVM.")
+else()
+    set(XGL_LLVM_SRC_PATH ${PROJECT_SOURCE_DIR}/../llvm CACHE PATH "Specify the path to the LLVM.")
+endif()
 
 # Wayland
 if (BUILD_WAYLAND_SUPPORT)