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

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

Differential Revision: https://reviews.llvm.org/D93498

mlir/include/mlir/Support/LLVM.h

@@ -23,6 +23,13 @@
 #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