[clang] [clang] Support --sysroot= for ${arch}-windows-msvc targets (PR #96417)

[Sharadh Rajaraman via cfe-commits]

sharadhr wrote:

> Let's do not make things worse by creating more fragmentations

But this PR does precisely that, by expecting a non-standard directory layout for `--sysroot`. By non-standard I mean modified from what the vendor provides. It might be a 'de-facto standard for UNIX' but it is _not_ the standard for Windows.

If the goal is to get `--sysroot` to expect `{arch}-pc-windows-msvc` then I believe the correct solution is to alias it to `-Xmicrosoft-windows-sys-root` when the argument value is `{arch}-pc-windows-msvc`, and ensure case-insensitivity is correctly handled in the driver. As it is, this PR is therefore incomplete.

https://github.com/llvm/llvm-project/pull/96417