Add basic reference tracking based on name lookups.

This tracks top-level qualified and unqualified lookups in the primary
source file, meaning we see all top-level names used in the file. This
is part of the intra-module dependency tracking work that can enable
incremental rebuilds.

This doesn't quite cover all of a file's dependencies. In particular, it
misses cases involving extensions  defined in terms of typealiases, and
it doesn't yet track operator lookups. The whole scheme is also very
dependent on being used to track file-level dependencies; if C is a subclass
of B and B is a subclass of A, C doesn't appear to depend on A. It only
works because changing A will mark B as dirty.

Part of rdar://problem/15353101

Swift SVN r22925

include/swift/Frontend/Frontend.h

@@ -269,7 +269,9 @@ class CompilerInstance {
   DiagnosticEngine Diagnostics{SourceMgr};
   std::unique_ptr<ASTContext> Context;
   std::unique_ptr<SILModule> TheSILModule;
+
   DependencyTracker *DepTracker = nullptr;
+  ReferencedNameTracker *NameTracker = nullptr;
 
   Module *MainModule = nullptr;
   SerializedModuleLoader *SML = nullptr;
@@ -293,6 +295,7 @@ class CompilerInstance {
   SourceFile *PrimarySourceFile = nullptr;
 
   void createSILModule();
+  void setPrimarySourceFile(SourceFile *SF);
 
 public:
   SourceManager &getSourceMgr() { return SourceMgr; }
@@ -316,6 +319,14 @@ public:
     return DepTracker;
   }
 
+  void setReferencedNameTracker(ReferencedNameTracker *tracker) {
+    assert(!PrimarySourceFile && "must be called before performSema()");
+    NameTracker = tracker;
+  }
+  ReferencedNameTracker *getReferencedNameTracker() {
+    return NameTracker;
+  }
+
   /// Set the SIL module for this compilation instance.
   ///
   /// The CompilerInstance takes ownership of the given SILModule object.

include/swift/Frontend/FrontendOptions.h

@@ -97,6 +97,9 @@ public:
   /// The path to which we should output a Make-style dependencies file.
   std::string DependenciesFilePath;
 
+  /// The path to which we should output a Swift reference dependencies file.
+  std::string ReferenceDependenciesFilePath;
+
   /// Arguments which should be passed in immediate mode.
   std::vector<std::string> ImmediateArgv;