Make sure we don't crash if someone (E.G.) comments out on entry from...

Make sure we don't crash if someone (E.G.) comments out on entry from g_core_definitions[] without removing the ArchSpec::Core enumeration when submitting from source.

We now catch the issue with a static_assert() at compile time and use llvm::array_lengthof(g_core_definitions) as well.

<rdar://problem/17767541>

llvm-svn: 213778