[clang] [Clang][Driver][LTO] Change the filename format for LTO'd stats file (PR #70242)

[Min-Yih Hsu via cfe-commits]

mshockwave wrote:

> I can understand the rationale, but adding this special case feels stranger to me..

I'm fine with not having a special file extension for LTO'd stats file, hence closing this PR.
That said, it would be really helpful if you could help me to review a related PR #71197 . 

https://github.com/llvm/llvm-project/pull/70242