[llvm-ml] Add support for the .S extension

Even though MASM files typically have the .asm extension, there are some
use cases [0] where they have the .S extension. MSVC ml assembles such
files with no problems, so llvm-ml should as well.

Additionally, fix the implementation of the /Ta flag and add a test for
it.

[0]: https://crrev.com/c/3668287

Reviewed By: epastor

Differential Revision: https://reviews.llvm.org/D126425