llvm-cov: fix a typo and rename a variable.

Rename Size to EndPos, which makes more sense because the variable
stores the last location of the blocks.

Patch by Yuchen Wu!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193189 91177308-0d34-0410-b5e6-96231b3b80d8