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

tools/clang/PGO-stage2.cmake

Index: tools/clang/PGO-stage2.cmake
diff --git a/tools/clang/PGO-stage2.cmake b/tools/clang/PGO-stage2.cmake
new file mode 100644
index 0000000000000000000000000000000000000000..c00690a6071dc0f35751e5be4c19675587754179
--- /dev/null
+++ b/tools/clang/PGO-stage2.cmake
@@ -0,0 +1,15 @@
+# This file sets up a CMakeCache for PGO stage2 bootstrap.
+set(CMAKE_BUILD_TYPE RELEASE CACHE STRING "")
+set(LLVM_BUILD_EXTERNAL_COMPILER_RT ON CACHE BOOL "")
+set(LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")
+set(LLVM_ENABLE_THREADS OFF CACHE BOOL "")
+set(LLVM_USE_CRT_RELEASE MT CACHE STRING "")
+
+# Remove this once official builds pass -Wl,--build-id explicitly
+# https://crbug.com/622775
+set(ENABLE_LINKER_BUILD_ID ON CACHE BOOL "")

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

I have a patch to remove this at https://codereview.chromium.org/2784423003/ –
you don't need this.

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

I'll remove this one, no problems.

+
+set(LIBCXX_INSTALL_HEADERS ON CACHE BOOL "")
+set(LIBCXX_INCLUDE_TESTS OFF CACHE BOOL "")
+
+set(CHROMIUM_TOOLS_SRC ${CMAKE_CURRENT_LIST_DIR} CACHE STRING "")