`@section` rejects C function references as non-const

[grynspan]

jon@jgubuntu:~$ swift --version
Swift version 6.3-dev (LLVM bff1370bd79c983, Swift 57cf4ce)
Target: x86_64-unknown-linux-gnu
Build config: +assertions

Using @section to store a reference to a C function (declared in Swift as @convention(c)) fails. This worked with @_section. See: https://github.com/swiftlang/swift-testing/actions/runs/19238300266/job/54994093045?pr=1408

This does look like it works on Darwin, but fails on Linux.

[grynspan]

/swift/tmp/Sources/tmp/tmp.swift:1:66: error: not supported in a constant expression
1 | @section(".misc") let x: (UInt, @convention(c) () -> Void) = (0, {})
  |                                                                  `- error: not supported in a constant expression
2 | @section(".misc") let y: @convention(c) () -> Void = {}
3 | 

/swift/tmp/Sources/tmp/tmp.swift:2:54: error: not supported in a constant expression
1 | @section(".misc") let x: (UInt, @convention(c) () -> Void) = (0, {})
2 | @section(".misc") let y: @convention(c) () -> Void = {}
  |                                                      `- error: not supported in a constant expression

[grynspan]

/swift/tmp/Sources/tmp/tmp.swift:3:75: error: not supported in a constant expression
1 | struct S {
2 |   static let z: @convention(c) () -> Void = {}
3 |   @section(".misc") static let w: (UInt, @convention(c) () -> Void) = (0, z)
  |                                                                           `- error: not supported in a constant expression
4 | }

[grynspan]

static func references fail too because they cannot be implicitly converted to C functions (unlike free functions.)