Separate dependency tracking from Make-style .d file generation.

This refactoring is groundwork for saving the cross-module dependencies
in the swiftdeps files as well, so that we know to rebuild files if an
outside file changes (such as a bridging header, another framework's
headers, or another framework's swiftmodule).

Part of rdar://problem/19270920

Swift SVN r24258

lib/AST/ASTContext.cpp

@@ -39,7 +39,6 @@ using namespace swift;
 
 LazyResolver::~LazyResolver() = default;
 void ModuleLoader::anchor() {}
-void DependencyTracker::anchor() {}
 
 llvm::StringRef swift::getProtocolName(KnownProtocolKind kind) {
   switch (kind) {