Error related to ThinLTO caching needs to be downgraded to a remark

This is a fix for PR #46392 (Diagnostic message (error) related to
ThinLTO caching needs to be downgraded to a remark).

There are diagnostic messages related to ThinLTO caching that contain
the word "error", but they are really just notices/remarks for users,
and they don't cause a build failure. The word "error" appearing can be
confusing to users, and may even cause deeper problems.

User's build system might be designed to interpret any error messages
(even a benign error message as the one above) reported by the compiler
as a build failure, thus causing the build to fail "needlessly". In
short, the term "error" in this diagnostic is misleading at best, and
may be causing build systems to fail at worst.

Differential Revision: https://reviews.llvm.org/D82138