Workaround around clang 5.0 bug by including SmallVector.h in LLVM.h (PR41549)

[mehdi_amini]

Summary

The forward declaration for SmallVector does not play well with clang-5.

[mehdi_amini]

Update title: clang-5 is the buggy compiler this time

[silvas]

I think you also should do the same for

clang/include/clang/Basic/LLVM.h
lld/include/lld/Common/LLVM.h

Otherwise folks will this this same issue on the bots when SmallVector<T> starts being used there. (maybe change a random SmallVector<T, N> to a SmallVector<T> in each of those subprojects)

mlir/include/mlir/Support/LLVM.h

 #ifndef MLIR_SUPPORT_LLVM_H
 #define MLIR_SUPPORT_LLVM_H
 
 // We include these two headers because they cannot be practically forward
 // declared, and are effectively language features.
 #include "llvm/ADT/None.h"
 #include "llvm/Support/Casting.h"
 
+// Workaround for clang-5 (PR41549)
+#if defined(__clang_major__)
+#if __clang_major__ <= 5
+#include "llvm/ADT/SmallVector.h"
+#endif
+#endif
+
 // Forward declarations.
 namespace llvm {
 // String types
 template <unsigned N>
 class SmallString;
 class StringRef;
 class StringLiteral;
 class Twine;