[clang][cli] Pass '-Wspir-compat' to cc1 from driver

This patch moves the creation of the '-Wspir-compat' argument from cc1 to the driver.

Without this change, generating command line arguments from `CompilerInvocation` cannot be done reliably: there's no way to distinguish whether '-Wspir-compat' was passed to cc1 on the command line (should be generated), or if it was created within `CompilerInvocation::CreateFromArgs` (should not be generated).

This is also in line with how other '-W' flags are handled.

(This was introduced in D21567.)

Reviewed By: Anastasia

Differential Revision: https://reviews.llvm.org/D97041