[llvm-ml] Remove all file extension restrictions

After D126425 was submitted, hans@ observed that MSVC's ml.exe doesn't
care about the file's extension at all. Now, we check if the file exists
to determine whether an input filename is a valid assembly file.

To keep things consistent with clang-cl and lld-link, llvm-ml will treat
everything that's not a flag as a filename.

Reviewed By: hans

Differential Revision: https://reviews.llvm.org/D126931