Fast dependency scanning for Swift

Implement a new "fast" dependency scanning option,
`-scan-dependencies`, in the Swift frontend that determines all
of the source file and module dependencies for a given set of
Swift sources. It covers four forms of modules:

1) Swift (serialized) module files, by reading the module header
2) Swift interface files, by parsing the source code to find imports
3) Swift source modules, by parsing the source code to find imports
4) Clang modules, using Clang's fast dependency scanning tool

A single `-scan-dependencies` operation maps out the full
dependency graph for the given Swift source files, including all
of the Swift and Clang modules that may need to be built, such
that all of the work can be scheduled up front by the Swift
driver or any other build system that understands this
option. The dependency graph is emitted as JSON, which can be
consumed by these other tools.

include/swift/AST/ModuleLoader.h

@@ -23,6 +23,7 @@
 #include "swift/Basic/SourceLoc.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallSet.h"
+#include "llvm/ADT/StringSet.h"
 #include "llvm/ADT/TinyPtrVector.h"
 
 namespace llvm {
@@ -41,7 +42,10 @@ class ClangImporterOptions;
 class ClassDecl;
 class FileUnit;
 class ModuleDecl;
+class ModuleDependencies;
+class ModuleDependenciesCache;
 class NominalTypeDecl;
+class SourceFile;
 class TypeDecl;
 
 enum class KnownProtocolKind : uint8_t;
@@ -177,6 +181,12 @@ public:
   /// Discover overlays declared alongside this file and add infomation about
   /// them to it.
   void findOverlayFiles(SourceLoc diagLoc, ModuleDecl *module, FileUnit *file);
+
+  /// Retrieve the dependencies for the given, named module, or \c None
+  /// if no such module exists.
+  virtual Optional<ModuleDependencies> getModuleDependencies(
+      StringRef moduleName,
+      ModuleDependenciesCache &cache) = 0;
 };
 
 } // namespace swift