[cxx-interop] Allow initializing std::function from Swift closures

This adds a Swift initializer to instantiations of `std::function` that accepts a Swift closure with `@convention(c)`.

rdar://103979602

Author: egorzhdan

test/Interop/Cxx/stdlib/use-std-function.swift

@@ -13,15 +13,15 @@ import CxxStdlib
 
 var StdFunctionTestSuite = TestSuite("StdFunction")
 
-StdFunctionTestSuite.test("FunctionIntToInt init empty") {
+StdFunctionTestSuite.test("FunctionIntToInt.init()") {
   let f = FunctionIntToInt()
   expectTrue(isEmptyFunction(f))
 
   let copied = f
   expectTrue(isEmptyFunction(copied))
 }
 
-StdFunctionTestSuite.test("FunctionIntToInt call") {
+StdFunctionTestSuite.test("FunctionIntToInt.callAsFunction") {
   let f = getIdentityFunction()
   expectEqual(123, f(123))
 }