Parsing support for if expressions. (#2883)

We model `if a then b else` as a prefix operator for parsing precedence purposes. The rule that a statement starting with `if` is never an `if` expression is handled implicitly because the statement parser never invokes the expression parser for a statement starting with `if`.

This exposed a bug in our diagnosis of the whitespace rule for prefix operators, which was incorrectly being applied to non-symbolic operators in some cases, and was producing a bogus second diagnostic in some cases, which is also fixed here.

toolchain/semantics/semantics_handle.cpp

@@ -178,6 +178,21 @@ auto SemanticsHandleIfConditionStart(SemanticsContext& context,
   return context.TODO(parse_node, "HandleIfConditionStart");
 }
 
+auto SemanticsHandleIfExpression(SemanticsContext& context,
+                                 ParseTree::Node parse_node) -> bool {
+  return context.TODO(parse_node, "HandleIfExpression");
+}
+
+auto SemanticsHandleIfExpressionElse(SemanticsContext& context,
+                                     ParseTree::Node parse_node) -> bool {
+  return context.TODO(parse_node, "HandleIfExpressionElse");
+}
+
+auto SemanticsHandleIfExpressionThen(SemanticsContext& context,
+                                     ParseTree::Node parse_node) -> bool {
+  return context.TODO(parse_node, "HandleIfExpressionThen");
+}
+
 auto SemanticsHandleIfStatement(SemanticsContext& context,
                                 ParseTree::Node parse_node) -> bool {
   return context.TODO(parse_node, "HandleIfStatement");