Skip PGO if clang is too old, and add warnings for skipped PGO (dotnet/coreclr#12248)

Skip PGO if clang is too old, and add warnings for skipped PGO (dotnet/coreclr#12248)

Clang/LLVM cannot merge profile data generated by a newer version of
itself. Training currently requires 3.6, but we don't want to completely
break the build for 3.5. I'm adding a version check to allow non-PGO
release builds on 3.5, along with a warning message. I'm also updating
the LTO detection to print a warning message if it causes PGO to be
disabled.

Commit migrated from https://github.com/dotnet/coreclr/commit/94788b256bc2aaa930341e27dc5ae3baf3bbf8b5