Allow function conversions to @convention(c) in @section expressions

[kubamracek]

For Swift Testing's use of @section, but generally useful too, and function conversions like this are not problematic in terms of static initialization.

[kubamracek]

@swift-ci please test

[grynspan]

Given:

func x() {}

struct S {
  static func y() {}

  static let xPrime: @convention(c) () -> Void = x
  static let yPrime: @convention(c) () -> Void = y
}

The compiler rejects yPrime:

/swift/tmp/Sources/tmp/tmp.swift:7:50: error: a C function pointer can only be formed from a reference to a 'func' or a literal closure
5 | 
6 |   static let xPrime: @convention(c) () -> Void = x
7 |   static let yPrime: @convention(c) () -> Void = y
  |                                                  `- error: a C function pointer can only be formed from a reference to a 'func' or a literal closure
8 | }
9 | 

It is necessary to support @convention(c) closures here FBOFW.

[DougGregor]

Thank you, LGTM

[kubamracek]

@swift-ci please test