builder, cgo: support function definitions in CGo headers

For example, the following did not work before but does work with this
change:

    // int add(int a, int b) {
    //   return a + b;
    // }
    import "C"

    func main() {
        println("add:", C.add(3, 5))
    }

Even better, the functions in the header are compiled together with the
rest of the Go code and so they can be optimized together! Currently,
inlining is not yet allowed but const-propagation across functions
works. This should be improved in the future.

testdata/cgo/main.go

@@ -10,6 +10,7 @@ int mul(int, int);
 */
 import "C"
 
+// int headerfunc(int a) { return a + 1; }
 import "C"
 
 import "unsafe"
@@ -43,6 +44,9 @@ func main() {
 	println("variadic0:", C.variadic0())
 	println("variadic2:", C.variadic2(3, 5))
 
+	// functions in the header C snippet
+	println("headerfunc:", C.headerfunc(5))
+
 	// equivalent types
 	var goInt8 int8 = 5
 	var _ C.int8_t = goInt8

testdata/cgo/out.txt

@@ -15,6 +15,7 @@ callback 1: 50
 callback 2: 600
 variadic0: 1
 variadic2: 15
+headerfunc: 6
 bool: true true
 float: +3.100000e+000
 double: +3.200000e+000