Get options also from the default target (#6530)

This helps to address issue #4760.

The particular issue motivating this fix is that
IGlobalSession::parseCommandLineArguments uses a temporary compile request to parse
options.
The compile request only adds the OptionKind::ForceDXLayout (-fvk-use-dx-layout) to the
"current target" which may be the default target, which
IGlobalSession::parseCommandLineArguments didn't look for, meaning that options like
ForceDXLayout were just ignored.
This leads to test failures in tests that rely on this option, e.g.
tests/spirv/cbuffer-dx-layout-1.slang.

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