ExplicitModuleLoader: take a JSON file that specifies details of explicit Swift modules

Instead of taking paths of Swift module files from front-end command line
arguments, we should take a JSON file specifying details of explicit modules.
The advantages is (1) .swiftdoc and .swiftsourceinfo can be associated
with a .swiftmodule file, and (2) module names are explicitly used as
keys in the JSON input so we don't need to eagerly deserialize a .swiftmodule
file to collect the module name.

include/swift/Frontend/ModuleInterfaceLoader.h

@@ -150,6 +150,7 @@ public:
   create(ASTContext &ctx,
          DependencyTracker *tracker, ModuleLoadingMode loadMode,
          ArrayRef<std::string> ExplicitModulePaths,
+         StringRef ExplicitSwiftModuleMap,
          bool IgnoreSwiftSourceInfoFile);
 
   /// Append visible module names to \p names. Note that names are possibly