[Modules] Fix bug where header resolution in modules doesn't work when compiling with relative paths.

[akhuang]

Summary

Currently when searching for header files (with relative paths) we concatenate
the path to the module map with the header filename. However when
searching for system headers it seems like we should start from the
current directory and not the path to the module map.

No test added because I'm not sure there's a way to use relative paths
to system headers in the lit tests.

Bug: https://bugs.llvm.org/show_bug.cgi?id=47209

[akhuang]

add case for include_nexts; I don't entirely understand this part of the code so not sure if it's what we want, but it seems to make building with -no-canonical-prefixes happier

[akhuang]

undo previous change

clang/lib/Lex/HeaderSearch.cpp

for (const auto &IncluderAndDir : Includers) {
       // a container that could be reallocated across this call.
       //
       // If we have no includer, that means we're processing a #include
       // from a module build. We should treat this as a system header if we're
       // building a [system] module.
       bool IncluderIsSystemHeader =
           Includer ? getFileInfo(Includer).DirInfo != SrcMgr::C_User :
           BuildSystemModule;
-      if (Optional<FileEntryRef> FE = getFileAndSuggestModule(
-              TmpDir, IncludeLoc, IncluderAndDir.second, IncluderIsSystemHeader,
-              RequestingModule, SuggestedModule)) {
+
+      Optional<FileEntryRef> FE = getFileAndSuggestModule(
+          RequestingModule, SuggestedModule);
+
+      // If this is a system header, we should also search from the current
+      // working directory and not the directory of the module map.
+      if (!FE && IncluderIsSystemHeader)
+        FE = getFileAndSuggestModule(
+            Filename, IncludeLoc, IncluderAndDir.second, IncluderIsSystemHeader,
+            RequestingModule, SuggestedModule);
+
+      if (FE) {
         if (!Includer) {
           assert(First && "only first includer can have no file");
           return FE;
         }
 
         // Leave CurDir unset.
         // This file is a system header or C++ unfriendly if the old file is.
         //