CMakeLists: Deprecate using llvm-config to find llvm install path

[tstellar]

Summary

You can still specify an llvm-config to use by configuring with
-DLLVM_CONFIG=/path/to/llvm/config, but this method is now deprecated
and may be remove.

Users should be setting CMAKE_PREFIX_PATH to the install prefix
for their llvm install. Unless you are using the system llvm in
which case, cmake should be able to automatically find the install
prefix.

[beanz]

LGTM!

CMakeLists.txt

 cmake_minimum_required(VERSION 3.4.3)
 
 # If we are not building as a part of LLVM, build Clang as an
 # standalone project, using LLVM as an external library:
 if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )
   project(Clang)
 
+  set(LLVM_CONFIG "" CACHE PATH "DEPRECATED: path to llvm-config. You should set CMAKE_PREFIX_PATH to the install prefix for llvm")
   # Rely on llvm-config.
   set(CONFIG_OUTPUT)
-  find_program(LLVM_CONFIG "llvm-config")
   if(LLVM_CONFIG)
     message(STATUS "Found LLVM_CONFIG as ${LLVM_CONFIG}")
     set(CONFIG_COMMAND ${LLVM_CONFIG}
-      "--assertion-mode"
-      "--bindir"
-      "--libdir"
-      "--includedir"
-      "--prefix"
-      "--src-root"
       "--cmakedir")
     execute_process(
       COMMAND ${CONFIG_COMMAND}
       RESULT_VARIABLE HAD_ERROR
       OUTPUT_VARIABLE CONFIG_OUTPUT
     )
     if(NOT HAD_ERROR)
       string(REGEX REPLACE
         "[ \t]*[\r\n]+[ \t]*" ";"
         CONFIG_OUTPUT ${CONFIG_OUTPUT})
     else()
       string(REPLACE ";" " " CONFIG_COMMAND_STR "${CONFIG_COMMAND}")
       message(STATUS "${CONFIG_COMMAND_STR}")
       message(FATAL_ERROR "llvm-config failed with status ${HAD_ERROR}")
     endif()
-  else()
-    message(FATAL_ERROR "llvm-config not found -- ${LLVM_CONFIG}")
-  endif()
-
-  list(GET CONFIG_OUTPUT 0 ENABLE_ASSERTIONS)
-  list(GET CONFIG_OUTPUT 1 TOOLS_BINARY_DIR)
-  list(GET CONFIG_OUTPUT 2 LIBRARY_DIR)
-  list(GET CONFIG_OUTPUT 3 INCLUDE_DIR)
-  list(GET CONFIG_OUTPUT 4 LLVM_OBJ_ROOT)
-  list(GET CONFIG_OUTPUT 5 MAIN_SRC_DIR)
-  list(GET CONFIG_OUTPUT 6 LLVM_CONFIG_CMAKE_PATH)
-
-  if(NOT MSVC_IDE)
-    set(LLVM_ENABLE_ASSERTIONS ${ENABLE_ASSERTIONS}
-      CACHE BOOL "Enable assertions")
-    # Assertions should follow llvm-config's.
-    mark_as_advanced(LLVM_ENABLE_ASSERTIONS)
-  endif()
-
-  set(LLVM_TOOLS_BINARY_DIR ${TOOLS_BINARY_DIR} CACHE PATH "Path to llvm/bin")
-  set(LLVM_LIBRARY_DIR ${LIBRARY_DIR} CACHE PATH "Path to llvm/lib")
-  set(LLVM_BINARY_DIR ${LLVM_OBJ_ROOT} CACHE PATH "Path to LLVM build tree")
-
+    set(LLVM_CONFIG_CMAKE_PATH ${CONFIG_OUTPUT})
     # Normalize LLVM_CMAKE_PATH. --cmakedir might contain backslashes.
     # CMake assumes slashes as PATH.
     file(TO_CMAKE_PATH ${LLVM_CONFIG_CMAKE_PATH} LLVM_CMAKE_PATH)
-
-  find_program(LLVM_TABLEGEN_EXE "llvm-tblgen" ${LLVM_TOOLS_BINARY_DIR}
-    NO_DEFAULT_PATH)
+  endif()
 
   find_package(LLVM REQUIRED HINTS "${LLVM_CMAKE_PATH}")
+  message(STATUS "LLVM install found at ${LLVM_DIR}")
   list(APPEND CMAKE_MODULE_PATH ${LLVM_DIR})
 
   set(LLVM_MAIN_INCLUDE_DIR ${LLVM_BINARY_DIR}/include)
 
+  find_program(LLVM_TABLEGEN_EXE "llvm-tblgen" ${LLVM_TOOLS_BINARY_DIR}
+    NO_DEFAULT_PATH)
+
   # They are used as destination of target generators.
   set(LLVM_RUNTIME_OUTPUT_INTDIR ${CMAKE_BINARY_DIR}/${CMAKE_CFG_INTDIR}/bin)
   set(LLVM_LIBRARY_OUTPUT_INTDIR ${CMAKE_BINARY_DIR}/${CMAKE_CFG_INTDIR}/lib${LLVM_LIBDIR_SUFFIX})
   if(WIN32 OR CYGWIN)
     # DLL platform -- put DLLs into bin.
     set(LLVM_SHLIB_OUTPUT_INTDIR ${LLVM_RUNTIME_OUTPUT_INTDIR})
   else()
     set(LLVM_SHLIB_OUTPUT_INTDIR ${LLVM_LIBRARY_OUTPUT_INTDIR})