Merging r347262:

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r347262 | vedantk | 2018-11-19 12:10:22 -0800 (Mon, 19 Nov 2018) | 8 lines

[Coverage] Fix PR39258: support coverage regions that start deeper than they end

popRegions used to assume that the start location of a region can't be
nested deeper than the end location, which is not always true.

Patch by Orivej Desh!

Differential Revision: https://reviews.llvm.org/D53244
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llvm-svn: 347798