Revert r231508 "CodeGen: Emit constant temporaries into read-only globals."

This broke the Chromium build. Links were failing with messages like:

obj/dbus/libdbus_test_support.a(obj/dbus/dbus_test_support.mock_object_proxy.o):../../dbus/mock_object_proxy.cc:function dbus::MockObjectProxy::Detach(): warning: relocation refers to discarded section
/usr/local/google/work/chromium/src/third_party/binutils/Linux_x64/Release/bin/ld.gold: error: treating warnings as errors

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@231541 91177308-0d34-0410-b5e6-96231b3b80d8