[NFC] Adjust SmallVector.h header to workaround XL build compiler issue

[Xiangling_L]

Summary

Previously delivered a patch to workaround the building issue related to SmallVector are here:
https://reviews.llvm.org/D98265
https://reviews.llvm.org/rG8dc70bdcd0fe4efb65876dce0144d9c3386a2f07

But in order to prevent further building issues related to the usage of it in other compilation unit, this patch adjusts the llvm.h header as a workaround instead.

We will further investigate the real problem of build compiler as a follow-up.

[jsji]

If we use this workaround, can we also revert 561fb7f60ab631e712c3fb6bbeb47061222c6818 and 8dc70bdcd0fe4efb65876dce0144d9c3386a2f07 in this patch?

[RKSimon]

clang/include/clang/Basic/LLVM.h

25

Is there a better comment? "unblock the XL build compiler issue" isn't going to mean much to someone in the future....

38

Should these 2 forward declarations be removed?

[Xiangling_L]

In D98552#2623399, @jsji wrote:

If we use this workaround, can we also revert 561fb7f60ab631e712c3fb6bbeb47061222c6818 and 8dc70bdcd0fe4efb65876dce0144d9c3386a2f07 in this patch?

Good idea. Will do

[Xiangling_L]

clang/include/clang/Basic/LLVM.h

25

How about "Add this header as a workaround to prevent "too few template arguments for class template 'SmallVector' ""? Or do you have any suggestion?

38

Thanks. will do.

[Xiangling_L]

Addressed the comments;

[hubert.reinterpretcast]

Thanks @Xiangling_L and also to the other reviewers for the great comments. This LGTM.

[Xiangling_L]

clang/include/clang/Basic/LLVM.h

71

It's my bad to not wait for a build finished before updating the patch, I saw a lot of build failures as the following that seems related to the deletion here:

/data/xling/wyvern/master-external/llvm-project/clang/include/clang/Basic/Module.h:222:3: error: no template named 'SmallVector'; did you mean 'llvm::SmallVector'?
  SmallVector<UnresolvedHeaderDirective, 1> UnresolvedHeaders;
  ^~~~~~~~~~~
  llvm::SmallVector
/data/xling/wyvern/master-external/llvm-project/clang/include/clang/Basic/LLVM.h:38:42: note: 'llvm::SmallVector' declared here
  template<typename T, unsigned N> class SmallVector;
                                         ^

I will update the patch accordingly after I have a successful build and run the test.

[Xiangling_L]

Bring back two forward declarations to avoid changing pervasive SmallVector to llvm::SmallVector

[hubert.reinterpretcast]

Retained using declarations look fine. LGTM.

clang/include/clang/Basic/LLVM.h

 
 // Do not proliferate #includes here, require clients to #include their
 // dependencies.
 // Casting.h has complex templates that cannot be easily forward declared.
 #include "llvm/Support/Casting.h"
 // None.h includes an enumerator that is desired & cannot be forward declared
 // without a definition of NoneType.
 #include "llvm/ADT/None.h"
+// Add this header as a workaround to unblock the XL build compiler issue.
+#include "llvm/ADT/SmallVector.h"
 
 namespace llvm {
   // ADT's.
   class StringRef;
   class Twine;
   class VersionTuple;
   template<typename T> class ArrayRef;
   template<typename T> class MutableArrayRef;
   template<typename T> class OwningArrayRef;
   template<unsigned InternalLen> class SmallString;
   template<typename T, unsigned N> class SmallVector;
   template<typename T> class SmallVectorImpl;
   template<typename T> class Optional;
   template <class T> class Expected;
 
   template<typename T>
   struct SaveAndRestore;
 
   // Reference counting.
   template <typename T> class IntrusiveRefCntPtr;
namespace clang {
   // ADT's.
   using llvm::ArrayRef;
   using llvm::MutableArrayRef;
   using llvm::None;
   using llvm::Optional;
   using llvm::OwningArrayRef;
   using llvm::SaveAndRestore;
   using llvm::SmallString;
   using llvm::SmallVector;
   using llvm::SmallVectorImpl;
   using llvm::StringRef;
   using llvm::Twine;
   using llvm::VersionTuple;
 
   // Error handling.
   using llvm::Expected;