[CMake] Use LLVM_DIR and Clang_DIR for standalone builds.

[JDevlieghere]

Summary

When doing a standalone build, without setting LLDB_PATH_TO_LLVM_BUILD or LLDB_PATH_TO_CLANG_BUILD, you get the following error.

CMake Error at cmake/modules/LLDBStandalone.cmake:23 (find_package):
  Could not find a package configuration file provided by "LLVM" with any of
  the following names:

    LLVMConfig.cmake
    llvm-config.cmake

  Add the installation prefix of "LLVM" to CMAKE_PREFIX_PATH or set
  "LLVM_DIR" to a directory containing one of the above files.  If "LLVM"
  provides a separate development package or SDK, be sure it has been
  installed.

This suggests setting LLVM_DIR to LLVM's install directory. However, LLDBStandalone.cmake takes LLDB_PATH_TO_LLVM_BUILD as its hint. As someone who isn't familiar with the standalone process, this is rather confusing. I propose removing LLDB_PATH_TO_LLVM_BUILD and LLDB_PATH_TO_CLANG_BUILD and use LLVM_DIR and Clang_DIR respectively.

[sgraenitz]

We shouldn't do the sole change here, but instead remove all LLDB_PATH_TO_<xy>_BUILD variables in favor of CMake standard <package>_DIR variables. Summarizing a discussion on this a while ago:

LLDB_PATH_TO_LLVM_BUILD used to be something like /path/to/llvm-build-tree
LLVM_DIR, however, is /path/to/llvm-build-tree/lib/cmake/llvm. Fine so far, but initializing..
LLDB_PATH_TO_CLANG_BUILD with the same value is incorrect, because it should be /path/to/llvm-build-tree/lib/cmake/clang!

In the past we also had (and downstream still have) things that would break, like:

set(CLANG_RESOURCE_PATH "${LLDB_PATH_TO_CLANG_BUILD}/lib${LLVM_LIBDIR_SUFFIX}/clang/${CLANG_VERSION}")

Fortunately most of it was cleaned up in the meantime. It looks like the only remaining blocker is the one below.

lldb/cmake/modules/LLDBStandalone.cmake

29–30

@xiaobai Can this be changed to something like ${LLVM_DIR}/../../../NATIVE?

[xiaobai]

lldb/cmake/modules/LLDBStandalone.cmake

29–30

Yeah I don't see why not. We could probably even change it to ${LLVM_BINARY_DIR}/NATIVE instead.

[JDevlieghere]

Use LLVM_DIR and Clang_DIR

lldb/cmake/modules/LLDBStandalone.cmake

 project(lldb)
 
 option(LLVM_INSTALL_TOOLCHAIN_ONLY "Only include toolchain files in the 'install' target." OFF)
 
-set(LLDB_PATH_TO_LLVM_BUILD "" CACHE PATH "Path to LLVM build tree")
-set(LLDB_PATH_TO_CLANG_BUILD "${LLDB_PATH_TO_LLVM_BUILD}" CACHE PATH "Path to Clang build tree")
-
-file(TO_CMAKE_PATH "${LLDB_PATH_TO_LLVM_BUILD}" LLDB_PATH_TO_LLVM_BUILD)
-file(TO_CMAKE_PATH "${LLDB_PATH_TO_CLANG_BUILD}" LLDB_PATH_TO_CLANG_BUILD)
-
-find_package(LLVM REQUIRED CONFIG
-  HINTS "${LLDB_PATH_TO_LLVM_BUILD}" NO_CMAKE_FIND_ROOT_PATH)
-find_package(Clang REQUIRED CONFIG
-  HINTS "${LLDB_PATH_TO_CLANG_BUILD}" NO_CMAKE_FIND_ROOT_PATH)
+find_package(LLVM REQUIRED CONFIG HINTS "${LLVM_DIR}" NO_CMAKE_FIND_ROOT_PATH)
+find_package(Clang REQUIRED CONFIG HINTS "${Clang_DIR}" NO_CMAKE_FIND_ROOT_PATH)
 
 # We set LLVM_CMAKE_PATH so that GetSVN.cmake is found correctly when building SVNVersion.inc
 set(LLVM_CMAKE_PATH ${LLVM_CMAKE_DIR} CACHE PATH "Path to LLVM CMake modules")
 
 set(LLVM_MAIN_SRC_DIR ${LLVM_BUILD_MAIN_SRC_DIR} CACHE PATH "Path to LLVM source tree")
 set(LLVM_MAIN_INCLUDE_DIR ${LLVM_MAIN_INCLUDE_DIR} CACHE PATH "Path to llvm/include")
 set(LLVM_BINARY_DIR ${LLVM_BINARY_DIR} CACHE PATH "Path to LLVM build tree")
 
 set(lit_file_name "llvm-lit")
 if(CMAKE_HOST_WIN32 AND NOT CYGWIN)
   set(lit_file_name "${lit_file_name}.py")
 endif()
 
 function(append_configuration_directories input_dir output_dirs)
   set(dirs_list ${input_dir})
   foreach(config_type ${LLVM_CONFIGURATION_TYPES})
     string(REPLACE ${CMAKE_CFG_INTDIR} ${config_type} dir ${input_dir})
     list(APPEND dirs_list ${dir})
   endforeach()
   set(${output_dirs} ${dirs_list} PARENT_SCOPE)
 endfunction()
 
 
 append_configuration_directories(${LLVM_TOOLS_BINARY_DIR} config_dirs)
 find_program(lit_full_path ${lit_file_name} ${config_dirs} NO_DEFAULT_PATH)
 set(LLVM_DEFAULT_EXTERNAL_LIT ${lit_full_path} CACHE PATH "Path to llvm-lit")
 
 if(CMAKE_CROSSCOMPILING)
-  set(LLVM_NATIVE_BUILD "${LLDB_PATH_TO_LLVM_BUILD}/NATIVE")
+  set(LLVM_NATIVE_BUILD "${LLVM_BINARY_DIR}/NATIVE")
   if (NOT EXISTS "${LLVM_NATIVE_BUILD}")
     message(FATAL_ERROR
       "Attempting to cross-compile LLDB standalone but no native LLVM build
       found. Please cross-compile LLVM as well.")
   endif()
 
   if (CMAKE_HOST_SYSTEM_NAME MATCHES "Windows")
     set(HOST_EXECUTABLE_SUFFIX ".exe")