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

[Ericson2314]

Summary

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.

[beanz]

LGTM

[Ericson2314]

(Note to posterity: I didn't wait for the build to finish because the last night's D116492 build already demonstrated this diff did not break things.)

clang/CMakeLists.txt

 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}")
     message(DEPRECATION "Using llvm-config to detect the LLVM installation is \
           deprecated.  The installed cmake files should be used \
           instead.  CMake should be able to detect your LLVM install \
           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.
     file(TO_CMAKE_PATH ${LLVM_CONFIG_CMAKE_DIR} LLVM_CMAKE_DIR)
   endif()
 
 
   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()
 
   find_package(LLVM REQUIRED HINTS "${LLVM_CMAKE_DIR}")
   list(APPEND CMAKE_MODULE_PATH ${LLVM_DIR})
 
   # We can't check LLVM_CONFIG here, because find_package(LLVM ...) also sets
   # LLVM_CONFIG.
   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)
 
   # 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)
if(LLVM_LIT)
       if( WIN32 AND NOT CYGWIN )
         set(LLVM_LIT_TOOLS_DIR "" CACHE PATH "Path to GnuWin32 tools")
       endif()
     else()
       set(LLVM_INCLUDE_TESTS OFF)
     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.
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")
 
 if(LLVM_ENABLE_LIBXML2)
   # Don't look for libxml if we're using MSan, since uninstrumented third party
   # code may call MSan interceptors like strlen, leading to false positives.