Introduce if/switch expressions

Introduce SingleValueStmtExpr, which allows the
embedding of a statement in an expression context.
This then allows us to parse and type-check `if`
and `switch` statements as expressions, gated
behind the `IfSwitchExpression` experimental
feature for now. In the future,
SingleValueStmtExpr could also be used for e.g
`do` expressions.

For now, only single expression branches are
supported for producing a value from an
`if`/`switch` expression, and each branch is
type-checked independently. A multi-statement
branch may only appear if it ends with a `throw`,
and it may not `break`, `continue`, or `return`.

The placement of `if`/`switch` expressions is also
currently limited by a syntactic use diagnostic.
Currently they're only allowed in bindings,
assignments, throws, and returns. But this could
be lifted in the future if desired.

lib/Sema/Constraint.cpp

@@ -1089,7 +1089,11 @@ Constraint *Constraint::createSyntacticElement(ConstraintSystem &cs,
                                                ContextualTypeInfo context,
                                                ConstraintLocator *locator,
                                                bool isDiscarded) {
+  // Collect type variables.
   SmallPtrSet<TypeVariableType *, 4> typeVars;
+  if (auto contextTy = context.getType())
+    contextTy->getTypeVariables(typeVars);
+
   unsigned size = totalSizeToAlloc<TypeVariableType *>(typeVars.size());
   void *mem = cs.getAllocator().Allocate(size, alignof(Constraint));
   return new (mem) Constraint(node, context, isDiscarded, locator, typeVars);