[libcxxabi] [cmake] Update LLVM_CMAKE_PATH following install layout change

[mgorny]

Summary

Update LLVM_CMAKE_PATH in stand-alone builds to match the new install layout used by LLVM 3.9+ where CMake files are installed into lib*/cmake/llvm rather than share/llvm/cmake.

[mgorny]

Independently fixed in rL283100.

CMakeLists.txt

elseif(LLVM_CONFIG_PATH)
 
     list(GET CONFIG_OUTPUT 0 INCLUDE_DIR)
     list(GET CONFIG_OUTPUT 1 LLVM_OBJ_ROOT)
     list(GET CONFIG_OUTPUT 2 MAIN_SRC_DIR)
 
     set(LLVM_INCLUDE_DIR ${INCLUDE_DIR} CACHE PATH "Path to llvm/include")
     set(LLVM_BINARY_DIR ${LLVM_OBJ_ROOT} CACHE PATH "Path to LLVM build tree")
     set(LLVM_MAIN_SRC_DIR ${MAIN_SRC_DIR} CACHE PATH "Path to LLVM source tree")
-    set(LLVM_CMAKE_PATH "${LLVM_BINARY_DIR}/share/llvm/cmake")
+    set(LLVM_CMAKE_PATH "${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}/cmake/llvm")
     set(LLVM_LIT_PATH "${LLVM_PATH}/utils/lit/lit.py")
   else()
     message(FATAL_ERROR "llvm-config not found and LLVM_MAIN_SRC_DIR not defined. "
                         "Reconfigure with -DLLVM_CONFIG_PATH=path/to/llvm-config "
                         "or -DLLVM_PATH=path/to/llvm-source-root.")
   endif()
 
   if(EXISTS ${LLVM_CMAKE_PATH})