llvm-cov: Prevent a test from matching its own check lines

Since llvm-cov shows the source file in its output, be careful about
potentially matching the check lines themselves.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218138 91177308-0d34-0410-b5e6-96231b3b80d8
llvm-cov: Return unique_ptrs instead of filling objects (NFC)

Having create* functions return the object they create is more
readable than using an in-out parameter.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218139 91177308-0d34-0410-b5e6-96231b3b80d8