[lldb][NFCI] Change logic to find clang resource dir in standalone builds

As of 0beffb8 there is a CMake
function to actually calculate the relative path to the clang resource
directory. Currently we have some bespoke logic that looks in a few
places, but with this new function we should be able to eliminate some
complexity here.

Also, I moved the functionality from LLDBConfig to LLDBStandalone since
it is only used in standalone builds.

Differential Revision: https://reviews.llvm.org/D156270

Author: bulbazord

lldb/cmake/modules/LLDBConfig.cmake

@@ -282,30 +282,6 @@ if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
   endif()
 endif()
 
-
-# If LLDB is building against a prebuilt Clang, then the Clang resource
-# directory that LLDB is using for its embedded Clang instance needs to point
-# to the resource directory of the used Clang installation.
-if (NOT TARGET clang-resource-headers)
-  set(LLDB_CLANG_RESOURCE_DIR_NAME "${LLVM_VERSION_MAJOR}")
-  # Iterate over the possible places where the external resource directory
-  # could be and pick the first that exists.
-  foreach(CANDIDATE "${Clang_DIR}/../.." "${LLVM_DIR}" "${LLVM_LIBRARY_DIRS}"
-                    "${LLVM_BUILD_LIBRARY_DIR}"
-                    "${LLVM_LIBRARY_DIR}")
-    # Build the resource directory path by appending 'clang/<version number>'.
-    set(CANDIDATE_RESOURCE_DIR "${CANDIDATE}/clang/${LLDB_CLANG_RESOURCE_DIR_NAME}")
-    if (IS_DIRECTORY "${CANDIDATE_RESOURCE_DIR}")
-      set(LLDB_EXTERNAL_CLANG_RESOURCE_DIR "${CANDIDATE_RESOURCE_DIR}")
-      break()
-    endif()
-  endforeach()
-
-  if (NOT LLDB_EXTERNAL_CLANG_RESOURCE_DIR)
-    message(FATAL_ERROR "Expected directory for clang-resource headers not found: ${LLDB_EXTERNAL_CLANG_RESOURCE_DIR}")
-  endif()
-endif()
-
 # Find Apple-specific libraries or frameworks that may be needed.
 if (APPLE)
   if(NOT APPLE_EMBEDDED)

lldb/cmake/modules/LLDBStandalone.cmake

@@ -128,3 +128,16 @@ endif()
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX})
 set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX})
+
+# If LLDB is building against a prebuilt Clang, then the Clang resource
+# directory that LLDB is using for its embedded Clang instance needs to point to
+# the resource directory of the used Clang installation.
+if (NOT TARGET clang-resource-headers)
+  include(GetClangResourceDir)
+  get_clang_resource_dir(LLDB_EXTERNAL_CLANG_RESOURCE_DIR
+    PREFIX "${Clang_DIR}/../../../")
+
+  if (NOT EXISTS ${LLDB_EXTERNAL_CLANG_RESOURCE_DIR})
+    message(FATAL_ERROR "Expected directory for clang-resource-headers not found: ${LLDB_EXTERNAL_CLANG_RESOURCE_DIR}")
+  endif()
+endif()