Issue Description:

When implementing PostfixIfConfigExprSyntax support for swift-format, I found that nested #if expressions are being returned as UnknownExprSyntax nodes rather than PostfixIfConfigExprSyntax. For example, in this snippet:

receiver
{{ #if ONE}}
{{ #if TWO}}
{{ .call()}}

The outer postfix expression (#if ONE) is PostfixIfConfigExprSyntax, but the inner one (#if TWO) is an UnknownExprSyntax. The inner node's children still appear to be correctly parsed as IfConfigDeclSyntax, however, at least in this simple case; only the expression's node itself was incorrect.

The above code snippet repro's with the syntax parser library built from main (as of 7/14) and also with the Xcode 13b2 compiler using -Xfrontend -emit-syntax:

parse.swift:3:9: error: unknown expression syntax exists in the source
{{ #if TWO}}
{{ ^}}