[Coverage] Restore the correct count value after processing a nested region in...

[Coverage] Restore the correct count value after processing a nested region in case of combined regions.

If several regions cover the same area of code, we have to restore
the combined value for that area when return from a nested region.

This patch achieves that by combining regions before calling buildSegments.

Differential Revision: http://reviews.llvm.org/D18610

llvm-svn: 267390