Add O API to slang_version, and auto convert it to 24.

  - No new runtime APIs added in O. We should keep the bitcode version
  to be 24.

Bug: 63141957
Test: mm and manually invoked llvm-rs-cc with -target-api 26 and
      verified that it is converted to 24.

Change-Id: Ie9366718393a6a76e6557ac9f05f9877670b1b9d
(cherry picked from commit 38d179cec358bd910586d6672cf2ebea6d8283c2)
(cherry picked from commit 183064027e2ca516e225ad50470a67a899e4262a)
2 files changed