Fix llvm-cov counts to be 64-bit integers to avoid overflows.

Line counts in llvm-cov are read in as 64-bit integers but were being truncated
to 32-bit in collectLineCounts(), which caused overflow for large counts.
This patch fixes all counts to be uint64_t.

Patch by Yuchen Wu!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193172 91177308-0d34-0410-b5e6-96231b3b80d8