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/Unicode.cpp

@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "swift/Basic/Assertions.h"
 #include "swift/Basic/Unicode.h"
 #include "swift/Basic/Compiler.h"
 #include "llvm/ADT/SmallString.h"