[Clang][Type] Add static assertion to guard future expansion for BuiltinType numbering

Recent expansion D152070 exceeds the limit and I had a hard time
triaging the bug because the overflow just creates unexpected
behaviors within the compiler.

This patch adds a static assertion to keep an eye for overflows when we
expand more types in the future.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D152429