[NFC] Include SmallVector.h for Clang <= 5

Clang 5 (possibly earlier) has a bug where it does not merge default
template arguments properly for forwarded declarations. Include
`SmallVector.h` directly in this case. This will slightly increase
compile times, but only for very old compilers and only for files that
don't already include `SmallVector.h` (of which there's currently only
23).

Author: bnbarham

include/swift/Basic/LLVM.h

@@ -25,6 +25,14 @@
 // None.h includes an enumerator that is desired & cannot be forward declared
 // without a definition of NoneType.
 #include "llvm/ADT/None.h"
+#if defined(__clang_major__) && __clang_major__ < 6
+// Add this header as a workaround to prevent `too few template arguments for
+// class template 'SmallVector'` on the buggy Clang 5 compiler (it doesn't
+// merge template arguments correctly). Remove once the CentOS 7 job is
+// replaced.
+// rdar://98218902
+#include "llvm/ADT/SmallVector.h"
+#endif
 
 // Don't pre-declare certain LLVM types in the runtime, which must
 // not put things in namespace llvm for ODR reasons.