[CMake] Abstract Config.h generation for Xcode

This patch abstracts the generation of Config.h and creates a dummy project entry point to allow generation of LLDB's Config header without performing a full CMake configuration.

This will enable the Xcode project to generate LLDB's Config header.

llvm-svn: 301553
diff --git a/lldb/CMakeLists.txt b/lldb/CMakeLists.txt
index fe640fe..ada2938 100644
--- a/lldb/CMakeLists.txt
+++ b/lldb/CMakeLists.txt
@@ -1,8 +1,15 @@
 cmake_minimum_required(VERSION 3.4.3)
 
-include(cmake/modules/LLDBStandalone.cmake)
-include(cmake/modules/LLDBConfig.cmake)
-include(cmake/modules/AddLLDB.cmake)
+# Add path for custom modules
+set(CMAKE_MODULE_PATH
+  ${CMAKE_MODULE_PATH}
+  "${CMAKE_CURRENT_SOURCE_DIR}/cmake"
+  "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules"
+  )
+
+include(LLDBStandalone)
+include(LLDBConfig)
+include(AddLLDB)
 
 if (CMAKE_SYSTEM_NAME MATCHES "Windows|Android")
   set(LLDB_DEFAULT_DISABLE_LIBEDIT 1)