Make assignment an expression.

Change AssignStmt into AssignExpr; this will make assignment behave more consistently with assignment-like operators, and is a first step toward integrating '=' parsing with SequenceExpr resolution so that '=' can obey precedence rules. This also nicely simplifies the AST representation of c-style ForStmts; the initializer and increment need only be Expr* instead of awkward Expr*/AssignStmt* unions.

This doesn't actually change any user-visible behavior yet; AssignExpr is still only parsed at statement scope, and typeCheckAssignment is still segregrated from the constraint checker at large. (In particular, a PipeClosureExpr containing a single assign expr in its body still doesn't use the assign expr to resolve its own type.) The parsing issue will be addressed by handling '=' during SequenceExpr resolution. typeCheckAssignment can hopefully be reworked to work within the constraint checker too.

Swift SVN r5500

lib/AST/Verifier.cpp

@@ -276,7 +276,7 @@ namespace {
       return checkLValue(Dest->getType(), "LHS of assignment");
     }
 
-    void verifyChecked(AssignStmt *S) {
+    void verifyChecked(AssignExpr *S) {
       Type lhsTy = checkAssignDest(S->getDest());
       checkSameType(lhsTy, S->getSrc()->getType(), "assignment operands");
     }