[clang] Fix or emit diagnostic for checked arithmetic builtins with

_ExtInt types

- Fix computed size for _ExtInt types passed to checked arithmetic
  builtins.
- Emit diagnostic when signed _ExtInt larger than 128-bits is passed
    to __builtin_mul_overflow.
- Change Sema checks for builtins to accept placeholder types.

Differential Revision: https://reviews.llvm.org/D81420