[clang] [Clang] Unescape doubled backslashes in #line filenames (PR #215992)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

Generally, I'm opposed to this change because there is very little logic to it.
There is no reason for us to treat `\\` and `\t` any differently.
If we decide that line directives are not escaped, we should be consistent about it. especially given the fact `\\` in some windows network paths etc might be semantically meaningful, collapsing them might do more harm than good.

https://github.com/llvm/llvm-project/pull/215992