[CMake] Fix libc++abi standalone cmake build.

[logan]

Summary

The cmake files install directory has been changed to
${prefix}/lib/cmake/llvm since r259821. Searching cmake modules in
${prefix}/share/llvm/cmake will result in fatal errors.

This commit fixes the out-of-tree build by changing the CMake module
search path to: "$(llvm-config --obj-root)/lib/cmake/llvm"

[logan]

Thanks for reviewing. Committed as 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})