[SR-9316] libSyntax: @convention function type is not supported in all contexts

[dan-zheng]

Previous ID	SR-9316
Radar	None
Original Reporter	@dan-zheng
Type	Bug

Environment

Swift built from this commit: ab37099

Additional Detail from JIRA

Votes	0
Component/s	Source Tooling
Labels	Bug, Syntax
Assignee	None
Priority	Medium

md5: 34a2083c388b96b7ef7fc883dadbf305

Issue Description:

Simple reproducer:

// convention.swift
// This passes syntax verification.
let f: @convention(c) (Int) -> Int = { x in x }

func foo<T>(_ x: T.Type) {}
// This does not.
foo((@convention(c) (Int) -> Int).self)

$ swiftc -Xfrontend -verify-syntax-tree convention.swift
convention.swift:6:6: error: unknown expression syntax exists in the source
foo((@convention(c) (Int) -> Int).self)
     ^

[dan-zheng]

This bug blocks the following PR: #20674

[belkadan]

cc @rintaro

[AnthonyLatsis]

libSyntax was removed in #62145 and succeeded by swift-syntax.