Emit warning when umbrella directory does not exists

[yamaguchi]

Summary

This is a patch for [1].

In /lib/Lex/ModuleMap.cpp, it was checking if umbrella directory exists, and emitted error if the directory doesn't exists.
So I changed error to warning, because in [1] discussion it was said "this should probably be at most a warning".

However, I'm not sure which file to add test case, so I would like to ask for an advice.

[1] https://bugs.llvm.org/show_bug.cgi?id=30508

[v.g.vassilev]

The test case seems at a reasonable place to me.

[v.g.vassilev]

LGTM!

[v.g.vassilev]

Landed in r300594.

include/clang/Basic/DiagnosticLexKinds.td

 def note_mmap_lsquare_match : Note<"to match this ']'">;
 def err_mmap_expected_member : Error<
   "expected umbrella, header, submodule, or module export">;
 def err_mmap_expected_header : Error<"expected a header name after '%0'">;
 def err_mmap_expected_mmap_file : Error<"expected a module map file name">;
 def err_mmap_module_redefinition : Error<
   "redefinition of module '%0'">;
 def note_mmap_prev_definition : Note<"previously defined here">;
-def err_mmap_umbrella_dir_not_found : Error<
-  "umbrella directory '%0' not found">;
 def err_mmap_umbrella_clash : Error<
   "umbrella for module '%0' already covers this directory">;
 def err_mmap_module_id : Error<
   "expected a module name or '*'">;
 def err_mmap_expected_library_name : Error<
   "expected %select{library|framework}0 name as a string">;
 def err_mmap_config_macro_submodule : Error<
   "configuration macros are only allowed in top-level modules">;

 def warn_auto_module_import : Warning<
   "treating #%select{include|import|include_next|__include_macros}0 as an "
   "import of module '%1'">, InGroup<AutoImport>, DefaultIgnore;
 def note_implicit_top_level_module_import_here : Note<
   "submodule of top-level module '%0' implicitly imported here">;
 def warn_uncovered_module_header : Warning<
   "umbrella header for module '%0' does not include header '%1'">, 
   InGroup<IncompleteUmbrella>;
+def warn_mmap_umbrella_dir_not_found : Warning<
+  "umbrella directory '%0' not found">,
+  InGroup<IncompleteUmbrella>;
 def err_expected_id_building_module : Error<
   "expected a module name in '__building_module' expression">;
 def warn_use_of_private_header_outside_module : Warning<
   "use of private header from outside its module: '%0'">,
   InGroup<DiagGroup<"private-header">>, DefaultError;
 def err_undeclared_use_of_module : Error<
   "module %0 does not depend on a module exporting '%1'">;
 def warn_non_modular_include_in_framework_module : Warning<

lib/Lex/ModuleMap.cpp

   else {
     SmallString<128> PathName;
     PathName = Directory->getName();
     llvm::sys::path::append(PathName, DirName);
     Dir = SourceMgr.getFileManager().getDirectory(PathName);
   }
   
   if (!Dir) {
-    Diags.Report(DirNameLoc, diag::err_mmap_umbrella_dir_not_found)
+    Diags.Report(DirNameLoc, diag::warn_mmap_umbrella_dir_not_found)
       << DirName;
-    HadError = true;
     return;
   }
 
   if (UsesRequiresExcludedHack.count(ActiveModule)) {
     // Mark this header 'textual' (see doc comment for
     // ModuleMapParser::UsesRequiresExcludedHack). Although iterating over the
     // directory is relatively expensive, in practice this only applies to the
     // uncommonly used Tcl module on Darwin platforms.

test/Modules/umbrella-header-include-builtin.mm

 // REQUIRES: system-darwin
 
 // RUN: rm -rf %t
 // RUN: %clang -cc1 -fsyntax-only -nostdinc++ -isysroot %S/Inputs/libc-libcxx/sysroot -isystem %S/Inputs/libc-libcxx/sysroot/usr/include/c++/v1 -F%S/Inputs/libc-libcxx/sysroot/Frameworks -std=c++11 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -x objective-c++ %s
 
 #include <A/A.h>
+
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: echo "module NonExistent1 { umbrella \"NonExistent\" }" > %t/modules.modulemap
+// RUN: echo "" > %t/A.h
+// RUN: echo "#include \"A.h\"  int i;" > %t/T.cxx
+// RUN: %clang -I %t -fmodules -fsyntax-only %t/T.cxx
+// expected-warning {{ umbrella directory }}