[DirectX] Only embed dxil when writing object files

When emitting assembly we don't particularly want the binary DXIL
embedded in the output. This was mostly there for testing purposes, so
we update those tests to run the test directly using `opt` and
restrict the -dxil-embed and -dxil-globals passes to running normally
only in the case where we're trying to emit a DXContainer.

Differential Revision: https://reviews.llvm.org/D158051