[Coverage] Clarify ownership of a MemoryBuffer in the reader (NFC)

Pass a `MemoryBuffer &` to BinaryCoverageReader::create() instead of a
`std::unique_ptr<MemoryBuffer> &`. This makes it easier to reason about
the ownership of the buffer at a glance.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273326 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed