This patch fixes a buglet in the function body importer. Add hooks for
    keeping a stack of blocks corresponding to the block nesting in the
    imported function. This ensures that local variables and temps wind up
    correctly scoped and don't introduce collisions.
    
    New test case for this problem in CL 186717.
    
    Fixes golang/go#33158.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/186757

From-SVN: r273577