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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@282422 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 64f3b51479d689f9f313d5ab4b621cd31376049f)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm