Improve the diagnostic for #include_next occurring in a file not found
in the include path.

Instead of making the incorrect claim that the included file has an
absolute path, describe the actual problem: the including file was found
either by absolute path, or relative to such a file, or relative to the
primary source file.

llvm-svn: 356712
diff --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp
index 179b0cb..1850af9 100644
--- a/clang/lib/Lex/PPDirectives.cpp
+++ b/clang/lib/Lex/PPDirectives.cpp
@@ -2085,6 +2085,10 @@
     LookupFromFile = CurPPLexer->getFileEntry();
     Lookup = nullptr;
   } else if (!Lookup) {
+    // The current file was not found by walking the include path. Either it
+    // is the primary file (handled above), or it was found by absolute path,
+    // or it was found relative to such a file.
+    // FIXME: Track enough information so we know which case we're in.
     Diag(IncludeNextTok, diag::pp_include_next_absolute_path);
   } else {
     // Start looking up in the next directory.