Allow module aliases to be expressed in the explicit Swift module map JSON file.

For build systems that already generate these files, it makes sense to include the aliases so that the map file serves as a comprehensive index of how the module inputs are referenced.

include/swift/AST/ASTContext.h

@@ -615,6 +615,10 @@ public:
   /// are the real (physical) module names on disk.
   void setModuleAliases(const llvm::StringMap<StringRef> &aliasMap);
 
+  /// Adds a given alias to the map of Identifiers between module aliases and
+  /// their actual names.
+  void addModuleAlias(StringRef moduleAlias, StringRef realName);
+
   /// Look up option used in \c getRealModuleName when module aliasing is applied.
   enum class ModuleAliasLookupOption {
     alwaysRealName,