Fix is_msvc and use_hot_reload variables #1303 
     Merged  
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Extracted from #1289
Fixes #1297
Fixes two variables that weren't loading their values correctly:
is_msvcanduse_hot_reload. The former didn't properly apply todisable_exceptionsbecause it was previously evaluated before the windows target was parsed, so the expression was relocated with no further changes. The latter failed to evaluate the conditional properly, even if a value was provided via cla, so its fallback was changed toNone& the logic was migrated to just before the variable is first used.