[CMake] Use LIST_SEPARATOR rather than escaping in ExternalProject_Add

[phosek]

Summary

Escaping ; in list arguments passed to ExternalProject_Add doesn't seem
to be working in newer versions of CMake (see
https://public.kitware.com/Bug/view.php?id=16137 for more details). Use
a custom LIST_SEPARATOR instead which is the officially supported way.

[phosek]

Can you please take a look at this one as well (this one is Clang, the other one was LLVM)?

[beanz]

LGTM!

cfe/trunk/CMakeLists.txt

if (CLANG_ENABLE_BOOTSTRAP)
   endif()
 
   # Find all variables that start with BOOTSTRAP_ and populate a variable with
   # them.
   get_cmake_property(variableNames VARIABLES)
   foreach(variableName ${variableNames})
     if(variableName MATCHES "^BOOTSTRAP_")
       string(SUBSTRING ${variableName} 10 -1 varName)
-      string(REPLACE ";" "\;" value "${${variableName}}")
+      string(REPLACE ";" "," value "${${variableName}}")
       list(APPEND PASSTHROUGH_VARIABLES
         -D${varName}=${value})
     endif()
     if(${variableName} AND variableName MATCHES "LLVM_EXTERNAL_.*_SOURCE_DIR")
       list(APPEND PASSTHROUGH_VARIABLES
         -D${variableName}=${${variableName}})
     endif()
   endforeach()
 
   # Populate the passthrough variables
   foreach(variableName ${CLANG_BOOTSTRAP_PASSTHROUGH} ${_BOOTSTRAP_DEFAULT_PASSTHROUGH})
     if(DEFINED ${variableName})
       if("${${variableName}}" STREQUAL "")
         set(value "")
       else()
-        string(REPLACE ";" "\;" value ${${variableName}})
+        string(REPLACE ";" "," value "${${variableName}}")
       endif()
       list(APPEND PASSTHROUGH_VARIABLES
         -D${variableName}=${value})
     endif()
   endforeach()
 
   ExternalProject_Add(${NEXT_CLANG_STAGE}
     DEPENDS clang-bootstrap-deps
CMAKE_ARGS
                  -DCLANG_STAGE=${NEXT_CLANG_STAGE}
                 ${COMPILER_OPTIONS}
                 ${LTO_LIBRARY} ${LTO_AR} ${LTO_RANLIB} ${verbose} ${PGO_OPT}
     INSTALL_COMMAND ""
     STEP_TARGETS configure build
     USES_TERMINAL_CONFIGURE 1
     USES_TERMINAL_BUILD 1
     USES_TERMINAL_INSTALL 1
+    LIST_SEPARATOR ,
     )
 
   # exclude really-install from main target
   set_target_properties(${NEXT_CLANG_STAGE} PROPERTIES _EP_really-install_EXCLUDE_FROM_MAIN On)
   ExternalProject_Add_Step(${NEXT_CLANG_STAGE} really-install
     COMMAND ${CMAKE_COMMAND} --build <BINARY_DIR> --target install
     COMMENT "Performing install step for '${NEXT_CLANG_STAGE}'"
     DEPENDEES build