llvm: Add missing `VirtualFileSystem.h` include (#171848)
`vfs::FileSystem` is forward-declared in `SanitizerBinaryMetadata.h`.
The corresponding header must be included in any source file that
includes that header, or we risk issues when building with
`LLVM_BUILD_LLVM_DYLIB` to build LLVM as a DLL on Windows.
This effort is tracked in #109483.
