[clang] [clang] Delay normalization of `-fmodules-cache-path` (PR #150123)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

The `clang-scan-deps` issues on Windows were caused by a mismatch between what scanner passed to `DepFS->setBypassedPathPrefix()` (as-written on command line) vs what the rest of the compiler was asking from the VFS (normalized). I extracted the normalization into `CompilerInstance` and used it in all places necessary. This also made the change in `ASTWriter` from `AddString` to `AddPath` redundant, so I reverted that and the new implementation of path relativization. I can bring that OOB fix back in a follow up PR. Requesting another round of reviews.

https://github.com/llvm/llvm-project/pull/150123