[cmake] Use LLVM_CMAKE_PATH for GetSVN script

Use the LLVM_CMAKE_PATH variable to locate the GetSVN.cmake script.
The variable was already available in stand-alone builds, and is also
set by LLVM since r284581.

llvm-svn: 284584

lldb/source/CMakeLists.txt

@@ -53,7 +53,7 @@ endforeach()
 
 if(DEFINED lldb_vc)
   set(version_inc "${CMAKE_CURRENT_BINARY_DIR}/SVNVersion.inc")
-  set(get_svn_script "${LLVM_MAIN_SRC_DIR}/cmake/modules/GetSVN.cmake")
+  set(get_svn_script "${LLVM_CMAKE_PATH}/GetSVN.cmake")
 
   # Create custom target to generate the VC revision include.
   add_custom_command(OUTPUT "${version_inc}"