Add --enable-pgo option to build clang with PGO

tools/clang/CMakeLists.txt

 cmake_minimum_required(VERSION 2.8.8)
 project(ChromeExtras)
 enable_testing()
 
 list(APPEND CMAKE_MODULE_PATH "${LLVM_BUILD_DIR}/share/llvm/cmake")
 
 # These tools are built using LLVM's build system, not Chromium's.
 # The build script generates a shim CMakeLists.txt in the LLVM source tree,
 # which simply forwards to this file.
 
@@ skipping 40 matching lines @@
 
 function(cr_install)
   install(${ARGN} COMPONENT chrome-tools OPTIONAL)
 endfunction(cr_install)
 
 # Custom install target, so the chrome tools can be installed without installing
 # all the other LLVM targets.
 add_custom_target(cr-install COMMAND
   ${CMAKE_COMMAND} -D COMPONENT=chrome-tools -P cmake_install.cmake)
 
-foreach(tool ${CHROMIUM_TOOLS})
-  add_subdirectory(${tool})
-endforeach(tool)
+if (CHROMIUM_TOOLS)

[Nico, 2017/04/07 15:58:53]

why this change? seems independent of the rest of the patch (?)

[matthewtff.asm, 2017/04/11 12:14:43]

We do not pass -DBOOTSTRAP_CHROMIUM_TOOLS to cmake,
so it fails to build instrumented clang(which is between bootstrapped clang and
stage2 clang).

+  # CHROMIUM_TOOLS are separated by comma. To make it iterable cmake list
+  # replace those with semicolon.
+  string(REPLACE "," ";" CHROMIUM_TOOLS_LIST ${CHROMIUM_TOOLS})
+  foreach(tool ${CHROMIUM_TOOLS_LIST})
+    add_subdirectory(${tool})
+  endforeach(tool)
+endif(CHROMIUM_TOOLS)