Temporary fix: capture macros.unit to ensure we are using the right MeasureUnit is used with initIsBaseUnit

[iCharlesHu]

For some reason the unit being passed to unitIsBaseUnit is the same object MeasureUnit but from a different image (the system ICU). We work around this issue by explicitly capture macros.unit in a variable first, which seems to force the compiler to use the same symbol from the package.

This is a temporary fix.

[itingliu]

This commit message is no longer true as @jmschonfeld found out. Can we update it?

[itingliu]

For more context about this change - I think we now have evidence that this happens if the client app (in our case, XCTest) loads system Foundation, which loads system ICU, and results in two sets of ICU objects at run time.

[iCharlesHu]

Switching to swift-testing does seem to workaround this issue (swift-testing does not should not (please don't) link clang module Foundation). I'll remove this workaround once we switch to swift-testing.