Auto-upgrade can be expensive, especially UpgradeDebugInfo() since it runs the verifier.
With this patch, we can specify that the imported bitcode is built with the same revision of LLVM, meaning there's no need to run any auto-upgrading. For now, limit this to just debug info since projects like Rust want to support multiple versions of LLVM at the same time (at compiler build time, not at Rust source code build time) and run a subset of the autoupgrade functionality for simplicity.
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D143229
