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/parser/parser_state.def

@@ -181,7 +181,11 @@ CARBON_PARSER_STATE_VARIANTS2(Designator, Expression, Struct)
 
 // Handles processing of an expression.
 //
-// If valid prefix operator:
+// If `If`:
+//   1. Expression
+//   2. ExpressionThen
+//   3. ExpressionIfFinish
+// Else if valid prefix operator:
 //   1. Expression
 //   2. ExpressionLoopForPrefix
 // Else:
@@ -242,6 +246,29 @@ CARBON_PARSER_STATE(ExpressionLoopForBinary)
 //   1. ExpressionLoop
 CARBON_PARSER_STATE(ExpressionLoopForPrefix)
 
+// Completes an IfExpression.
+//
+// Always:
+//   (state done)
+CARBON_PARSER_STATE(ExpressionIfFinish)
+
+// Handles the `then` token in an `if` expression.
+//
+// If `Then`:
+//   1. Expression
+//   2. ExpressionElse
+// Else:
+//  (state done)
+CARBON_PARSER_STATE(ExpressionThen)
+
+// Handles the `else` token in an `if` expression.
+//
+// If `Else`:
+//   1. Expression
+// Else:
+//  (state done)
+CARBON_PARSER_STATE(ExpressionElse)
+
 // Handles the `;` for an expression statement, which is different from most
 // keyword statements.
 //