Sema: Type checking for C function pointers.

Since a function pointer doesn't carry any context, we can only form a C function pointer from a static reference to a global function or a context-free local function or closure. (Or maybe a static function applied to its metatype, but that's not handled here yet.) As a placeholder for a to-be-bikeshedded surface syntax, let the existing @cc(cdecl) attribute appear in source text when the -enable-c-function-pointers frontend flag is passed.

Swift SVN r25308

test/Parse/c_function_pointers.swift

@@ -0,0 +1,44 @@
+// RUN: %target-swift-frontend -parse -verify -module-name main -enable-c-function-pointers %s
+func global() -> Int { return 0 }
+
+struct S {
+  static func staticMethod() -> Int { return 0 }
+}
+
+class C {
+  static func staticMethod() -> Int { return 0 }
+  class func classMethod() -> Int { return 0 }
+}
+
+if true {
+  var x = 0
+  func local() -> Int { return 0 }
+  func localWithContext() -> Int { return x }
+
+  let a: @cc(cdecl) () -> Int = global
+  let a2: @cc(cdecl) () -> Int = main.global
+  let b: @cc(cdecl) () -> Int = { 0 }
+  let c: @cc(cdecl) () -> Int = local
+
+  // Can't convert a closure with context to a C function pointer
+  let d: @cc(cdecl) () -> Int = { x } // expected-error{{cannot be formed from a closure that captures context}}
+  let e: @cc(cdecl) () -> Int = localWithContext // expected-error{{cannot be formed from a local function that captures context}}
+
+  // Can't convert a closure value to a C function pointer
+  let global2 = global
+  let f: @cc(cdecl) () -> Int = global2 // expected-error{{can only be formed from a reference to a 'func' or a literal closure}}
+  let globalBlock: @objc_block () -> Int = global
+  let g: @cc(cdecl) () -> Int = globalBlock // expected-error{{can only be formed from a reference to a 'func' or a literal closure}}
+
+  // Can convert a function pointer to a block or closure, or assign to another
+  // C function pointer
+  let h: @cc(cdecl) () -> Int = a
+  let i: @objc_block () -> Int = a
+  let j: () -> Int = a
+
+  // Can't convert a C function pointer from a method.
+  // TODO: Could handle static methods.
+  let k: @cc(cdecl) () -> Int = S.staticMethod // expected-error{{}}
+  let m: @cc(cdecl) () -> Int = C.staticMethod // expected-error{{}}
+  let n: @cc(cdecl) () -> Int = C.classMethod // expected-error{{}}
+}