Strip local symbols when using externalized debug info.

When we build LLVM with externalized debug info, all debugging and
symbolication related data is extracted into dSYM files prior to
stripping. As such, there is no need to preserve local symbols in LLVM
binaries after dSYM creation.

This shrinks libLLVM.dylib from 58MB to 55MB on my system.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@258566 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/modules/AddLLVM.cmake

@@ -1191,7 +1191,7 @@ function(llvm_externalize_debuginfo name)
     endif()
     add_custom_command(TARGET ${name} POST_BUILD
       COMMAND xcrun dsymutil $<TARGET_FILE:${name}>
-      COMMAND xcrun strip -Sl $<TARGET_FILE:${name}>)
+      COMMAND xcrun strip -Sxl $<TARGET_FILE:${name}>)
   else()
     message(FATAL_ERROR "LLVM_EXTERNALIZE_DEBUGINFO isn't implemented for non-darwin platforms!")
   endif()