[clang] [clang] Delay checking of `-fopenmp-host-ir-file-path` (PR #150124)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

> LGTM. Ideally we should have a test that you can VFS map this file

That's a good point. I realized this PR switches `sys::fs::exists` to `vfs::FileSystem::exists` but not `MemoryBuffer::getFile` to `vfs::FileSystem::getBufferForFile` which isn't right. I'll switch both places to use `FileSystem` in a follow up PR and I'll keep this one small and only deal with moving the check out of command line parsing.

https://github.com/llvm/llvm-project/pull/150124