Make ModuleDependencyCollector's method virtual (NFC)

For reproducers in LLDB we want to hook up into the existing clang
infrastructure. To make that happen we need to be able to override the
ModuleDependencyCollector's methods.

The alternative was to inherit from the DependencyCollector directly,
but that would mean re-implementing the ModuleDependencyListener and the
ModuleDependencyPPCallbacks and ModuleDependencyMMCallbacks.

Differential revision: https://reviews.llvm.org/D58072

llvm-svn: 353882