[modules] Track how 'header' directives were written in module map files,

rather than trying to extract this information from the FileEntry after the
fact.

This has a number of beneficial effects. For instance, diagnostic messages for
failed module builds give a path relative to the "module root" rather than an
absolute file path, and the contents of the module includes file is no longer
dependent on what files the including TU happened to inspect prior to
triggering the module build.

llvm-svn: 223095

clang/lib/Frontend/CompilerInstance.cpp

@@ -1552,7 +1552,7 @@ CompilerInstance::loadModule(SourceLocation ImportLoc,
 
     // Check whether this module is available.
     clang::Module::Requirement Requirement;
-    clang::Module::HeaderDirective MissingHeader;
+    clang::Module::UnresolvedHeaderDirective MissingHeader;
     if (!Module->isAvailable(getLangOpts(), getTarget(), Requirement,
                              MissingHeader)) {
       if (MissingHeader.FileNameLoc.isValid()) {
@@ -1581,9 +1581,16 @@ CompilerInstance::loadModule(SourceLocation ImportLoc,
                      Module, ImportLoc);
   }
 
+  // Determine whether we're in the #include buffer for a module. The #includes
+  // in that buffer do not qualify as module imports; they're just an
+  // implementation detail of us building the module.
+  bool IsInModuleIncludes = !getLangOpts().CurrentModule.empty() &&
+                            getSourceManager().getFileID(ImportLoc) ==
+                                getSourceManager().getMainFileID();
+
   // If this module import was due to an inclusion directive, create an 
   // implicit import declaration to capture it in the AST.
-  if (IsInclusionDirective && hasASTContext()) {
+  if (IsInclusionDirective && hasASTContext() && !IsInModuleIncludes) {
     TranslationUnitDecl *TU = getASTContext().getTranslationUnitDecl();
     ImportDecl *ImportD = ImportDecl::CreateImplicit(getASTContext(), TU,
                                                      ImportLoc, Module,