[libLTO][AIX] Respect `-f[no]-integrated-as` on AIX

`libLTO` currently ignores the `-f[no-]integrated-as` flags. This patch teaches `libLTO` to respect them on AIX.

The implementation consists of two parts:

  # Migrate `llc`'s `-no-integrated-as` option to a codegen option so that the option is available to `libLTO`/`lld`/`gold`.
  # Teach `clang` to pass `-no-integrated-as` accordingly to `libLTO` depending on the `-f[no-]integrated-as` flags.

On platforms other than AIX, the `-f[no-]integrated-as` flags are ignored.

Reviewed By: MaskRay, steven_wu

Differential Revision: https://reviews.llvm.org/D152924