[ADT] Fix implicit reliance on cassert in StringTable.h (llvm#169324)

Adds an explicit include of `<cassert>` in StringTable.h rather than
relying on the one in StringRef.h. Fixes potential compile errors if
assert() was undef'ed between StringRef.h and StringTable.h inclusion.

Author: danilaml

llvm/include/llvm/ADT/StringTable.h

@@ -11,6 +11,7 @@
 
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/iterator.h"
+#include <cassert>
 #include <iterator>
 #include <limits>