Add #include "swift/Basic/Assertions.h" to a lot of source files

Although I don't plan to bring over new assertions wholesale
into the current qualification branch, it's entirely possible
that various minor changes in main will use the new assertions;
having this basic support in the release branch will simplify that.
(This is why I'm adding the includes as a separate pass from
rewriting the individual assertions)

lib/Basic/StringExtras.cpp

@@ -15,6 +15,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "swift/Basic/Assertions.h"
 #include "swift/Basic/StringExtras.h"
 #include "clang/Basic/CharInfo.h"
 #include "llvm/ADT/ArrayRef.h"