[clang] [flang] [flang] Add -fdisable-real-16 option (PR #221904)

[Shunsuke Watanabe via cfe-commits]

s-watanabe314 wrote:

> If a user explicitly uses REAL(16). say in a variable declaration, what effect will this option have?

Any explicit use of `REAL(16)` becomes a compile-time error. As shown in `disable-real-16.f90`, when `-fdisable-real-16` is specified, `REAL(16)` is treated as unsupported regardless of the target architecture, so declarations such as `REAL(16) :: x` are always rejected.

https://github.com/llvm/llvm-project/pull/221904