Serialize explicit module dependencies in swift module files

For clients, such as the debugger, who do not have access the full
output of the dependency scanner, it is a huger performance and
correctness improvement if each explicitly built Swift module not just
serialized all its Clang .pcm dependencies (via the serialized Clang
compiler invocation) but also its direct Swift module dependencies.

This patch changes the Swift module format to store the absolute path
or cas cache key for each dependency in the INPUT block, and makes
sure the deserialization makes these available to the ESML.

rdar://150969755

lib/Serialization/ModuleFile.cpp

@@ -210,6 +210,10 @@ ModuleFile::loadDependenciesForFileContext(const FileUnit *file,
     auto importPath = builder.copyTo(ctx);
     auto modulePath = importPath.getModulePath(dependency.isScoped());
     auto accessPath = importPath.getAccessPath(dependency.isScoped());
+    if (!getContext().LangOpts.DisableDeserializationOfExplicitPaths &&
+        !dependency.Core.BinaryModulePath.empty())
+      ctx.addExplicitModulePath(modulePath.front().Item.str(),
+                                dependency.Core.BinaryModulePath.str());
 
     auto module = getModule(modulePath, /*allowLoading*/true);
     if (!module || module->failedToLoad()) {