[llvm-cov] Silence a warning from the MSVC runtime (NFC)

Rework getLongestCommonPrefixLen() so that it doesn't access string null
terminators. The old version with std::mismatch would do this:

                        |
                        v
    Strings[0] = ['a', nil]

    Strings[1] = ['a', 'a', nil]
                        ^
                        |

This should silence a warning from the MSVC runtime (PR30515). As
before, I tested this out by preparing a coverage report for FileCheck.
Thanks to Yaron Keren for the report!

llvm-svn: 282422