[DependencyCollector] Make maybeAddDependency virtual (NFC)
[DependencyCollector] Make maybeAddDependency virtual (NFC)

Make DependencyCollector::maybeAddDependency, just like its other
methods, which I made virtual a while ago. The motivation for this
change is still the LLDB reproducer.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@367271 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit cb15cf86d9f70d93d4778b85f970346280bb0978)

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang