[clang][cmake] Rearrange top-level CMakeLists.txt for D116492

In that revision, I make LLD match Clang in deprecating `llvm-config`.
This patch isn't to worthwhile on its own --- there isn't a sense in
which the new order is "better" in isolation --- but by putting the
steps that LLD also neeeds to do first, I make the diff between LLD and
Clang's top-level `CMakeLists.txt` very legible.

Longer term I hope:

 1. We can remove calling `llvm-config` altogether, and just go strait to
    finding the CMake config file. This is what Flang does, at least.

 2. Hopefully the diffable part is smaller then --- i.e. there is less
    duplicated boilerplate.

 3. Any duplicate boilerplate that remains can be factored out.

I didn't both trying to factor anything out in e.g. the top level common
CMake Utility modules because this deprecated-but-not-removed state is a
merely transitional.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D116548

clang/CMakeLists.txt

@@ -2,15 +2,15 @@ cmake_minimum_required(VERSION 3.13.4)
 
 # 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 )
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   project(Clang)
 
   set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ standard to conform to")
   set(CMAKE_CXX_STANDARD_REQUIRED YES)
   set(CMAKE_CXX_EXTENSIONS NO)
 
   # Rely on llvm-config.
-  set(CONFIG_OUTPUT)
+  set(LLVM_CONFIG_OUTPUT)
   if(LLVM_CONFIG)
     set (LLVM_CONFIG_FOUND 1)
     message(STATUS "Found LLVM_CONFIG as ${LLVM_CONFIG}")
@@ -20,35 +20,36 @@ if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )
           automatically, but you can also use LLVM_DIR to specify \
           the path containing LLVMConfig.cmake.")
     set(CONFIG_COMMAND ${LLVM_CONFIG}
-      "--assertion-mode"
-      "--bindir"
-      "--libdir"
       "--includedir"
       "--prefix"
       "--src-root"
-      "--cmakedir")
+      "--cmakedir"
+      "--bindir"
+      "--libdir"
+      "--assertion-mode"
+      )
     execute_process(
       COMMAND ${CONFIG_COMMAND}
       RESULT_VARIABLE HAD_ERROR
-      OUTPUT_VARIABLE CONFIG_OUTPUT
+      OUTPUT_VARIABLE LLVM_CONFIG_OUTPUT
     )
     if(NOT HAD_ERROR)
       string(REGEX REPLACE
         "[ \t]*[\r\n]+[ \t]*" ";"
-        CONFIG_OUTPUT ${CONFIG_OUTPUT})
+        LLVM_CONFIG_OUTPUT ${LLVM_CONFIG_OUTPUT})
     else()
       string(REPLACE ";" " " CONFIG_COMMAND_STR "${CONFIG_COMMAND}")
       message(STATUS "${CONFIG_COMMAND_STR}")
       message(FATAL_ERROR "llvm-config failed with status ${HAD_ERROR}")
     endif()
 
-    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_DIR)
+    list(GET LLVM_CONFIG_OUTPUT 0 MAIN_INCLUDE_DIR)
+    list(GET LLVM_CONFIG_OUTPUT 1 LLVM_OBJ_ROOT)
+    list(GET LLVM_CONFIG_OUTPUT 2 MAIN_SRC_DIR)
+    list(GET LLVM_CONFIG_OUTPUT 3 LLVM_CONFIG_CMAKE_DIR)
+    list(GET LLVM_CONFIG_OUTPUT 4 TOOLS_BINARY_DIR)
+    list(GET LLVM_CONFIG_OUTPUT 5 LIBRARY_DIR)
+    list(GET LLVM_CONFIG_OUTPUT 6 ENABLE_ASSERTIONS)
 
     # Normalize LLVM_CMAKE_DIR. --cmakedir might contain backslashes.
     # CMake assumes slashes as PATH.
@@ -71,17 +72,17 @@ if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )
   if (NOT LLVM_CONFIG_FOUND)
     # Pull values from LLVMConfig.cmake.  We can drop this once the llvm-config
     # path is removed.
+    set(MAIN_INCLUDE_DIR ${LLVM_INCLUDE_DIR})
+    set(LLVM_OBJ_DIR ${LLVM_BINARY_DIR})
     set(TOOLS_BINARY_DIR ${LLVM_TOOLS_BINARY_DIR})
     set(LIBRARY_DIR ${LLVM_LIBRARY_DIR})
-    set(INCLUDE_DIR ${LLVM_INCLUDE_DIR})
-    set(LLVM_OBJ_DIR ${LLVM_BINARY_DIR})
   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_MAIN_INCLUDE_DIR ${INCLUDE_DIR} CACHE PATH "Path to llvm/include")
+  set(LLVM_MAIN_INCLUDE_DIR ${MAIN_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_TOOLS_BINARY_DIR ${TOOLS_BINARY_DIR} CACHE PATH "Path to llvm/bin")
+  set(LLVM_LIBRARY_DIR ${LIBRARY_DIR} CACHE PATH "Path to llvm/lib")
 
   find_program(LLVM_TABLEGEN_EXE "llvm-tblgen" ${LLVM_TOOLS_BINARY_DIR}
     NO_DEFAULT_PATH)
@@ -184,11 +185,12 @@ if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )
     endif()
   endif()
 
-  set( CLANG_BUILT_STANDALONE 1 )
+  set(CLANG_BUILT_STANDALONE TRUE)
+
   set(BACKEND_PACKAGE_STRING "LLVM ${LLVM_PACKAGE_VERSION}")
 else()
   set(BACKEND_PACKAGE_STRING "${PACKAGE_STRING}")
-endif()
+endif() # standalone
 
 # Make sure that our source directory is on the current cmake module path so that
 # we can include cmake files from this directory.