[ORC][MachO] Make BuildVersionOpts::fromTriple result optional, add test.

Only platform specific darwin OS values (e.g. macosx, ios, watchos, ...) can be
mapped to an LC_BUILD_VERSION platform. For

[ORC][MachO] Make BuildVersionOpts::fromTriple result optional, add test.

Only platform specific darwin OS values (e.g. macosx, ios, watchos, ...) can be
mapped to an LC_BUILD_VERSION platform. For all other values return an empty
optional to indicate that the load command can't be constructed.

Also fixes the simulator conditions to return the correct platform, and adds a
testcase.

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