[clang] [llvm] Enable fexec-charset option (PR #138895)

[Abhina Sree via cfe-commits]

abhina-sree wrote:

> I _think_ that ASTContext::getPredefinedStringLiteralFromCache should also be updated (didn't find it in the follow-up PR). It handles __builtin_FILE / __builtin_FILE_NAME that I think should return strings in execution encoding.

Thanks for pointing that out, I've addressed this change in the other PR in this commit https://github.com/llvm/llvm-project/pull/169803/changes/94d4f3c35dc02facda97a8c2c556341951aae6d5

https://github.com/llvm/llvm-project/pull/138895