[AST] Intro a check for the export_as relationship between 2 modules

Author: xymus

include/swift/AST/Module.h

@@ -877,6 +877,10 @@ class ModuleDecl
   /// Returns the associated clang module if one exists.
   const clang::Module *findUnderlyingClangModule() const;
 
+  /// Does this module or the underlying clang module defines export_as with
+  /// a value corresponding to the \p other module?
+  bool isExportedAs(const ModuleDecl *other) const;
+
   /// Returns a generator with the components of this module's full,
   /// hierarchical name.
   ///

lib/AST/Module.cpp

@@ -2175,6 +2175,14 @@ const clang::Module *ModuleDecl::findUnderlyingClangModule() const {
   return nullptr;
 }
 
+bool ModuleDecl::isExportedAs(const ModuleDecl *other) const {
+  auto clangModule = findUnderlyingClangModule();
+  if (!clangModule)
+    return false;
+
+  return other->getRealName().str() == clangModule->ExportAsModule;
+}
+
 void ModuleDecl::collectBasicSourceFileInfo(
     llvm::function_ref<void(const BasicSourceFileInfo &)> callback) const {
   for (const FileUnit *fileUnit : getFiles()) {