The LLVM trunk has reached 4.0 and now __clang_major__ is not enough
to distinguish between old XCode Clang and the new 'real' Clang.
Using __apple_build_version__ allows to make this distinction.

BUG=chromium:631144

Change-Id: I0b6e46fddfe4f409c7b7e558bda34872e60ee2d9