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

lib/Frontend/CompilerInvocation.cpp

@@ -478,6 +478,10 @@ static bool ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
                           OPT_emit_dependencies,
                           OPT_emit_dependencies_path,
                           "d", false);
+  determineOutputFilename(Opts.ReferenceDependenciesFilePath,
+                          OPT_emit_reference_dependencies,
+                          OPT_emit_reference_dependencies_path,
+                          "swiftdeps", false);
   determineOutputFilename(Opts.SerializedDiagnosticsPath,
                           OPT_serialize_diagnostics,
                           OPT_serialize_diagnostics_path,