Revert r220174, "Always use -Wl,-gc-sections on our build."

It dropped required functions for plugins with gnu ld 2.20 and 2.21.

  Failing Tests (1):
      LLVM :: Feature/load_module.ll

  Hello: bin/opt: symbol lookup error: lib/LLVMHello.so: undefined symbol: _ZN4llvm11raw_ostream13write_escapedENS_9StringRefEb

  Failing Tests (1):
      Clang :: Frontend/plugins.c

  error: unable to load plugin 'lib/PrintFunctionNames.so': 'lib/PrintFunctionNames.so: undefined symbol: _ZN5clang15PluginASTAction6anchorEv'

I think we should inspect linker's version or behavior to introduce --gc-sections for --export-dynamic.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220198 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/modules/AddLLVM.cmake

@@ -141,18 +141,15 @@ function(add_llvm_symbol_exports target_name export_file)
 endfunction(add_llvm_symbol_exports)
 
 function(add_dead_strip target_name)
-  if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
-    # ld64's implementation of -dead_strip breaks tools that use plugins.
-    if(NOT LLVM_NO_DEAD_STRIP)
+  if(NOT LLVM_NO_DEAD_STRIP)
+    if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
       set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                    LINK_FLAGS " -Wl,-dead_strip")
+    elseif(NOT WIN32)
+      # Object files are compiled with -ffunction-data-sections.
+      set_property(TARGET ${target_name} APPEND_STRING PROPERTY
+                   LINK_FLAGS " -Wl,--gc-sections")
     endif()
-  elseif(NOT WIN32)
-    # Object files are compiled with -ffunction-data-sections.
-    # On ELF --gc-sections handles --export-dynamic correctly, so we can always
-    # use it.
-    set_property(TARGET ${target_name} APPEND_STRING PROPERTY
-                 LINK_FLAGS " -Wl,--gc-sections")
   endif()
 endfunction(add_dead_strip)