More correct comments transformation (#152)

More correct comments transformation was implemented.

Added processing of //go:linkname localname [importpath.name] directive, now localname is not renamed. This is safe and does not cause a name disclosure because the functions marked //linkname do not have a name in the resulting binary.

Added cgo directives support

Fixed filename leak protection for cgo

Part of #149

testdata/scripts/syntax.txt

@@ -1,19 +1,20 @@
 env GOPRIVATE='test/main,rsc.io/*'
 
-garble build
+garble build -tags directives
 exec ./main$exe
 cmp stderr main.stderr
 
-! binsubstr main$exe 'localName' 'globalConst' 'globalVar' 'globalType' 'valuable information' 'rsc.io'
+! binsubstr main$exe 'localName' 'globalConst' 'globalVar' 'globalType' 'valuable information' 'rsc.io' 'remoteIntReturn' 'intReturn'
+binsubstr main$exe 'magicFunc'
 
 [short] stop # no need to verify this with -short
 
-go build
+go build -tags directives
 exec ./main$exe
 cmp stderr main.stderr
 
 binsubstr main$exe 'globalVar' # 'globalType' only matches on go < 1.15
-! binsubstr main$exe 'localName' 'globalConst'
+! binsubstr main$exe 'localName' 'globalConst' 'remoteIntReturn' 'intReturn'
 
 
 -- go.mod --
@@ -73,6 +74,7 @@ func main() {
 	scopesTest()
 	println(quote.Go())
 	sub.Test()
+	sub.TestDirectives()
 }
 
 -- scopes.go --
@@ -122,6 +124,39 @@ func Test() {
 
 func noop(...interface{}) {}
 
+-- sub/directives.go --
+// +build directives
+
+package sub
+
+import (
+    _ "unsafe"
+    _ "test/main/sub/a"
+)
+
+//go:linkname remoteIntReturn a.magicFunc
+
+func remoteIntReturn() int
+
+//go:noinline
+func TestDirectives() {
+    if remoteIntReturn() != 42 {
+        panic("invalid result")
+    }
+}
+-- sub/a/directives.go --
+// +build directives
+
+package a
+
+import _ "unsafe"
+
+//go:linkname intReturn a.magicFunc
+
+//go:noinline
+func intReturn() int {
+    return 42
+}
 -- main.stderr --
 nil case
 {"Foo":3}